ES12: The 24th Annual Workshop on Recent Developments in Electronic Structure Theory June 5 - 8, 2012 — Wake Forest University TIMO THONHAUSER Department of Physics Wake Forest University

NATALIE HOLZWARTH Department of Physics Wake Forest University

AKBAR SALAM Department of Chemistry Wake Forest University

Cover graphics: HOMO orbital of a phenalenyl dimer — Brian Kolb ES12: The 24th Annual Workshop on Recent Developments in Electronic Structure Theory

June 5–8, 2012 Wake Forest University Winston-Salem, NC 27109, USA ii Contents

1 Welcome 1 1.1 Workshop Background ...... 1 1.2 Funding ...... 1 1.3 Conference Banquet ...... 2 1.4 Wireless Access ...... 2 1.5 About Wake Forest University ...... 2

2 Program 3

3 Abstracts 9 3.1 Abstracts of Invited Talks ...... 9 3.2 Abstracts of Posters ...... 35

4 List of Conference Participants 105

5 Index 113 5.1 Index of Invited Talks ...... 113 5.2 Index of Posters ...... 114

iii iv 1 Welcome

1.1 Workshop Background

The Annual Workshop on Recent Developments in Electronic Structure Theory brings to- gether active participants in electronic structure theory from universities, colleges, insti- tutes and laboratories around the world. The invited presentations and contributed posters describe new methods for computing previously inaccessible properties of materials, break- throughs in computational eciency and accuracy, and novel applications of these methods to the study of molecules, liquids, and solids. This conference represents a valuable oppor- tunity for students, postdocs, and senior researchers alike to present their ideas and learn from each other. This workshop was started by David M. Ceperley and Richard M. Martin (Physics, University of Illinois) in 1989. Previous workshops are listed at the Materials Computation Center Website of the University of Illinois. The 2012 workshop in this series is organized by Profs. Timo Thonhauser, Natalie Holzwarth, and Akbar Salam.

1.2 Funding

The ES12 workshop is made possible by the generous support of:

Wake Forest University, Oce of the Provost • Wake Forest University, Departments of Physics and Chemistry • U. S. Army Research Oce • U. S. Department of Energy • Materials Computation Center, University of Illinois, • Supported by NSF Award DMR 11-07472.

1 2012 Electronic Structure Workshop Wake Forest University

1.3 Conference Banquet

The conference banquet is kindly sponsored by the BOSCH Ab-Initio Materials Modeling Group, directed by Dr. Boris Kozinsky.

1.4 Wireless Access

The university o↵ers wireless access in all buildings throughout campus. To access the wireless network, please connect to the open guest network WFUvisitor, open your browser, and use the following credentials to log in: username ES12xW9t password n7r6kVVd ! ! 1.5 About Wake Forest University

Founded in 1834, Wake Forest University is a private, collegiate university dedicated to serving humanity through the pursuit of knowledge. Ranked among the nation’s top 25 universities, Wake Forest o↵ers students the personalized attention of a small liberal arts college with the resources, technology, and co-curricular opportunities of a large university. With strong undergraduate and graduate programs and leading professional schools, Wake Forest maintains the highest academic and scholarly standards and strives to instill in all its students a desire to lead examined and purposeful lives.

Location Winston-Salem, NC Founded 1834, Wake Forest, NC Status Private, Collegiate University President Nathan O. Hatch, A.B., A.M., Ph.D. Motto Pro Humanitate (for humanity) Enrollment 7,162 (2010–2011) Undergraduate 4,657 Graduate 2,505 Student/Faculty Ratio 11:1 Academic Majors 37

Wake Forest o↵ers more than 400 semester, summer, and year-long study abroad programs in 200 cities in more than 70 countries worldwide, including University owned residences in London, Venice, and Vienna. More than 60% of Wake Forest undergraduates take advantage of these opportunities. The student-run Volunteer Service Corps plans international service trips, furthering the University’s motto, Pro Humanitate.

2 2 Program

Registration

4:00 pm – 8:00 pm Tuesday, June 5, 2012 Polo Residence Hall 8:00 am – 12:30 pm Wednesday, June 6, 2012 Pugh Auditorium 2:00 pm – 6:00 pm Wednesday, June 6, 2012 Pugh Auditorium 8:00 am – 12:30 pm Thursday, June 7, 2012 Pugh Auditorium 2:00 pm – 6:00 pm Thursday, June 7, 2012 Pugh Auditorium

Tuesday, June 5, 2012

7:00 pm – 9:00 pm Welcome Reception Polo Residence Hall

3 2012 Electronic Structure Workshop Wake Forest University

Wednesday, June 6, 2012

7:00 am – 8:00 am Breakfast Fresh Food Company 8:15 am – 8:30 am Welcome and Opening Remarks Pugh Auditorium 8:30 am – 10:15 am Plenary Session I Pugh Auditorium

Challenging Materials Chair: Mei-Yin Chou, Academia Sinica

Oana Jurchescu, Wake Forest University Tailoring Crystalline Order in Organic Thin-Film Transistors by Exploring Interactions at Interfaces

Noa Marom, University of Texas at Austin Electronic Structure of Dye-Sensitized TiO2 Clusters from G0W0

Pieremanuele Canepa, Wake Forest University Sequestration and Di↵usion of Small Non-Polar Molecules in MOF Materials

10:15 am – 10:45 am Co↵ee Break Pugh Auditorium 10:45 am – 12:30 pm Plenary Session II Pugh Auditorium

Simulations of Phase Transitions Chair: David Ceperley, University of Illinois, Urbana-Champaign

Marc Torrent, CEA, France Dense Hydrogen by First-Principles Path-Integral Molecular Dynamics: Structure of Phase II, Melting Curve, and Beyond

Je↵McMahon, University of Illinois On the Molecular Dissociation of Dense Hydrogen and the Solid/Liquid Transition in the Atomic Phase via Free-energy Calculations

Daniel Sheppard, Los Alamos National Laboratory A Generalized Solid-State Nudged Elastic Band Method

12:30 pm – 2:00 pm Lunch Fresh Food Company 2:00 pm – 3:30 pm Poster Session A (even numbers) 401 Benson

4 2012 Electronic Structure Workshop Wake Forest University

3:45 pm – 5:30 pm Plenary Session III Pugh Auditorium

Large Scale Simulations and Materials Design Chair: Alan F. Wright, Sandia National Laboratories

David Bowler, University College London Recent Developments in the Linear Scaling DFT code CONQUEST: Constrained DFT, TDDFT, and Basis Sets

Joseph Bennett, Rutgers University The Discovery and Design of Multifunctional Materials: Integration of Database Searching and First Principles Calculations

Cai-Zhuang Wang, Ames Lab, Iowa State University Adaptive Genetic Algorithm Method for Crystal Structure Prediction and Materials Discovery

6:30 pm – 7:30 pm Cocktail Reception Green Room, Reynolda Hall 7:30 pm – 9:30 pm Conference Banquet Magnolia Room, Reynolda Hall

5 2012 Electronic Structure Workshop Wake Forest University

Thursday, June 7, 2012

7:00 am – 8:00 am Breakfast Fresh Food Company 8:30 am – 10:15 am Plenary Session IV Pugh Auditorium

Developments in Density Functional Theory Chair: Normand Modine, Sandia National Laboratory

Nicola Marzari, EPFL, Switzerland Density-Functional Theory: Time to Move on?

Andreas G¨orling, University of Erlangen-N¨urnberg, Germany The Adiabatic-Connection Dissipation-Fluctuation Theorem as Route to a New Generation of Density-Functional Methods

Rodney J. Bartlett, University of Florida Is There a Consistent Density Functional Theory?

10:15 am – 10:45 am Co↵ee Break Pugh Auditorium 10:45 am – 12:30 pm Plenary Session V Pugh Auditorium

Beyond DFT: GW, Time-Dependence, and EM Fields Chair: Jerry Bernholc, North Carolina State University

Josef Zwanziger, Dalhousie University, Canada Homogeneous Electric and Magnetic Fields in Periodic Systems

Stefano Baroni, SISSA, Italy Ab Initio Colors

Sohrab Ismail-Beigi, Yale University Progress and Challenges with Luttinger-Ward Approaches for Going Beyond DFT

12:30 pm – 2:00 pm Lunch Fresh Food Company 2:00 pm – 3:30 pm Poster Session B (odd numbers) 401 Benson

6 2012 Electronic Structure Workshop Wake Forest University

3:45 pm – 5:30 pm Plenary Session VI Pugh Auditorium

Quantum Monte Carlo Chair: Cyrus Umrigar, Cornell University

Claudia Filippi, University of Twente, The Netherlands Size-Extensive Wave Functions for Quantum Monte Carlo: A Linear Scaling Generalized Valence Bond Approach

Bryan Clark, Princeton University Approaching Strongly Correlated Systems using Partial Node FCIQMC

Shiwei Zhang, College of William and Mary Auxiliary-Field Quantum Monte Carlo Calculations of Excited States and Strongly Correlated Systems

6:00 pm – 7:30 pm Dinner Fresh Food Company 7:30 pm – 10:00 pm Community Panel Discussions Byrum Auditorium

7 2012 Electronic Structure Workshop Wake Forest University

Friday, June 8, 2012

7:00 am – 8:00 am Breakfast Fresh Food Company 8:30 am – 10:15 am Plenary Session VII Pugh Auditorium

Partitioning, Embedding, and Complex Systems Chair: Jeongnim Kim, Oak Ridge National Laboratory

Lai Jiang, University of Pennsylvania Electrons on a Leash: Topology-based Charge Partitioning and Rigorous Definition of Oxidation States in Solids

Garnet Chan, Princeton University Density Matrix Entanglement Embedding for Strongly Correlated Electronic Structure

Renata Wentzcovitch, University of Minnesota Spin Crossover Systems in the Deep Mantle

10:15 am – 10:45 am Co↵ee Break Pugh Auditorium 10:45 am – 12:30 pm Plenary Session VIII Pugh Auditorium

Topological Insulators Chair: David Vanderbilt, Rutgers University

Ra↵aele Resta, University of Trieste, Italy Topological Order in Electronic Wavefunctions

Andrew Rappe, University of Pennsylvania Dirac Semimetal in Three Dimensions

Alexey Soluyanov, Rutgers University First-Principles Calculation of Topological Invariants

12:30 pm – 12:45 pm Closing Remarks Pugh Auditorium 12:45 pm – 2:00 pm Lunch (lunch boxes available) Fresh Food Company

8 3 Abstracts

3.1 Abstracts of Invited Talks

Abstracts are sorted alphabetically by last name of the presenting author. If an abstract has several authors, the presenting author is listed first. An index of invited talks can be found in Section 5.1 on page 113.

9 2012 Electronic Structure Workshop Wake Forest University

Ab Initio Colors

S. Baroni,1 A. Calzolari,2 and R. Gebauer3

1SISSA, Scuola Internazionale Superiore di Studi Avanzati, Trieste, Italy. 2CNR-NANO, Istituto di Nanoscienze, Modena, Italy. 3ICTP, The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy.

In this talk I will present some recent work addressing the e↵ects of the solvent (water) on the optical properties of natural dyes. I will break the ice with a short presentation of the physics and physiology of color vision, in the style of popular science. I will then introduce Time-Dependent Density-Functional Perturbation Theory, a technique allowing for the simulation of the optical spectra of molecular models of up to several hundred (and possibly even a few thousand) inequivalent atoms, without computing any virtual states. This technique will be demonstrated with the “prediction” that grass is green, and applied to the optical properties of flavylium, the die that gives aubergines and blueberries their typical deep-purple coloration [1]. In the latter case I will show that the main e↵ect of the solvent is to provide a medium allowing thermal fluctuations to fill the gaps that would otherwise characterize the spectrum of the dye at T = 0, thus considerably enhancing optical absorption in the visible range.

[1] O. B. Malcioglu, A. Calzolari, R. Gebauer, D. Varsano, and S. Baroni, JACS 133, 15425 (2011).

Contact: [email protected]

10 2012 Electronic Structure Workshop Wake Forest University

Is There a Consistent Density Functional Theory?

R. J. Bartlett, A. Perera, and P. Verma

Quantum Theory Project, Department of Chemistry and Physics, University of Florida, Gainesville, FL 32611-8435, USA.

The meaning of a consistent KSDFT is one whose functional provides accurate total energies and related properties, while its potential, has the property that its eigenvalues are good approximations to all the principal ionization potentials of a molecule, not just the homo. There is no currently used KSDFT method that is consistent, except the ab initio dft method we have been pursuing. Hence, the latter demonstrates that such an e↵ective one- particle theory exists, and its numerical results for a variety of properties are superior to more conventional GGA, hybrid, and other KSDFT methods. In our opinion the weak link in current KSDFT is the inaccuracy of the potential, so we have addressed that issue several ways. We will discuss some of them here.

[1] P. Varma and R. J. Bartlett, J. Chem. Phys. 136, 044105 (2012). [2] P. Verma, A. Perera, and R. J. Bartlett, Chem. Phys. Letts. 524, 10 (2012). [3] R. J. Bartlett, Wiley Interdisciplinary Reviews – Computational Molecular Science 2, 126 (2011).

Contact: [email protected]fl.edu

11 2012 Electronic Structure Workshop Wake Forest University

The Discovery and Design of Multifunctional Materials: Integration of Database Searching and First Principles Calculations

J. W. Bennett

Department of Physics and Astronomy, The State University of New Jersey, NJ 08854-8019 USA.

As technological advances in device syntheses continue to accelerate at an increased rate, there arises an overwhelming need for multifunctional materials. In this talk, we will discuss the development of strategies for the integration of crystallographic database searching with high-throughput first-principles density functional methods for the discovery and design of novel multifunctional materials. We will focus upon recent studies of ternary ABC compositions, of which the most well-known examples are the half-Heusler family, and how this had led us to the discovery of a potential new class of ferroelectrics that is based upon an underdeveloped, previously overlooked family of known materials.

Contact: [email protected]

12 2012 Electronic Structure Workshop Wake Forest University

Recent Developments in the Linear Scaling DFT Code CONQUEST: Constrained DFT, TDDFT, and Basis Sets

D. R. Bowler,1,2,3 T. Miyazaki,4 C. O’Rourke,1,2,3 and M. Gillan1,2,3

3London Centre for Nanotechnology, UCL, London WC1H 0AH, UK. 2Department of Physics & Astronomy, UCL, London WC1E 6BT, UK. 3Thomas Young Centre, UCL, London WC1E 6BT, UK. 4National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan.

Linear scaling or O(N) electronic structure codes have been under development for around fifteen years [1]. After an initial explosion of interest, the practical diculties of implemen- tation and eciency have led to a slow down in development and applications. In this talk I will present details of recent developments in the massively parallel CONQUEST linear scaling DFT code, and make some comments on the linear scaling field in general. The CONQUEST code [2] is one of the leading O(N) codes, and has demonstrated not only excellent scaling to over two million atoms and many thousands of cores [3] but also practical applications to nanostructures on semiconductor surfaces [4], and recently to biological systems. I will describe the details of the CONQUEST code, including recent developments in basis functions and parallelisation. I will also discuss recent improvements including constrained DFT [5], exact exchange and TDDFT, all of which have been imple- mented with linear scaling.

This work was funded by the Royal Society, EPSRC, BBSRC and a Grant-in-Aid for Scientific Research from MEXT and JSPS, Japan.

[1] D. R. Bowler and T. Miyazaki, Rep. Prog. Phys. 75, 036503 (2012). [2] D. R. Bowler, R. Choudhury, M. J. Gillan, and T. Miyazaki, phys. stat. sol. b 243, 989 (2006). [3] D. R. Bowler and T. Miyazaki, J. Phys.: Condens. Matter 22, 074207 (2010). [4] T. Miyazaki, D. R. Bowler, M. J. Gillan, and T. Ohno, J. Phys. Soc. Jpn. 77, 123706 (2008). [5] A. M. P. Sena, T. Miyazaki, and D. R. Bowler, J. Chem. Theory Comput. 7, 884 (2011).

Contact: [email protected]

13 2012 Electronic Structure Workshop Wake Forest University

Sequestration and Di↵usion of Small Non-Polar Molecules in MOF Materials

P. Canepa,1 N. Nijem,2 Y. J. Chabal,2 and T. Thonhauser1

1Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA. 2Department of Materials Science & Engineering, University of Texas at Dallas, Richardson, TX 75080, USA.

Distressing scenarios of fossil fuel shortages and increasing green-house e↵ects have inten- sified scientific e↵orts to find innovative materials capable of ecient/e↵ective hydrogen storage and gas separation. Metal organic frameworks (MOF) are a broad class of novel porous materials that exhibit the necessary prerequisites for feasible hydrogen storage and gas separation. For example, MOFs have large surface areas, high porosity, high thermal stability, high structural flexibility, and easily tunable chemical and physical properties [1, 2, 3]. A great deal of experimental e↵ort has been made in synthesizing MOFs and tai- loring their chemical properties, but insight into the physisorption phenomena that govern MOF macroscopic properties is best obtained through a combination of experiment and theory. We will present promising results from such a combined study, focusing on theoret- ical DFT simulations of the adsorption and di↵usion of small, non-polar molecules such as H2, CO2, and N2 throughout the MOF sca↵old. Standard exchange-correlation functionals are not capable of describing weak van der Waals interactions responsible for binding these molecules inside MOFs; however, we will show that the truly non-local functional vdW- DF [4, 5] gives results—for a variety of properties of MOFs—in remarkable agreement with our experiments.

This work was supported in full by the DOE Oce of Basic Energy Sciences, Materials Sciences and Engineering Division, Grant No. DE-FG02-08ER46491.

[1] N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Kee↵e, and O. M. Yaghi, Science 300, 1127 (2003). [2] A. R. Millward and O. M. Yaghi, J. Am. Chem. Soc. 127, 17998 (2005). [3] E. D. Bloch, W. L. Queen, R. Krishna, J. M. Zadrozny, C. M. Brown, and J. R. Long, Science 335, 1606 (2012). [4] M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004); T. Thonhauser et al., Phys. Rev. B 76, 125112 (2007). [5] D. C. Langreth et al., J. Phys.: Condens. Matter 21, 084203 (2009).

Contact: [email protected]

14 2012 Electronic Structure Workshop Wake Forest University

Density Matrix Entanglement Embedding for Strongly Correlated Electronic Structure

G. K.-L. Chan and G. Knizia

Department of Chemistry, Princeton University, Princeton, NJ 08544, USA.

Self-consistent quantum embedding forms a route to bridge local descriptions of electronic structure methods and the electronic structure of very large systems and condensed phases. Existing examples of quantum embedding methods include the density functional embed- ding of Cortona, Wesolowski and Warshel, based on the single-particle density and Carter, and the dynamical mean-field theory, based on the single-particle Greens function. In this talk, however, I will describe a new formalism for quantum embedding that is based on the single-particle density matrix, and which is based on a self-consistent approximation to the exact entanglement embedding of a subsystem in an environment. Unlike density functional embedding, this formalism can practically describe strongly correlated systems, and unlike dynamical mean-field theory, it is easily combined with traditional ground-state electronic structure methods, such as quantum chemical wavefunction methods and projector and variational Monte Carlo approaches. I will demonstrate the performance of the method on the phase diagram of the 1D and 2D Hubbard models and discuss applications to ab-initio systems.

Contact: [email protected]

15 2012 Electronic Structure Workshop Wake Forest University

Approaching Strongly Correlated Systems Using Partial Node FCIQMC

B. K. Clark1,2 and M. Kolodrubetz1

1Department of Physics, Princeton University, Princeton, NJ, USA. 2Princeton Center for Theoretical Science, Princeton, NJ, USA.

Finding the ground state of a fermionic Hamiltonian is a dicult problem, due to the Fermion sign problem. While still scaling exponentially, full configuration-interaction Monte Carlo (FCI-QMC) [1] mitigates some of the exponential variance by allowing annihilation of noise – whenever two walkers arrive at the same configuration with opposite signs, they are removed from the simulation. While FCI-QMC has been quite successful for quantum chemistry problems, it has had less success with strongly correlated systems; for example, it is unstable for even small systems sizes when applied to the one of the simplest many-body systems: the Fermi-Polaron. In this talk, we explore a series of algorithmic improvements which significantly increase its e↵ectiveness. These improvements include modifying fixed node to work in this method, developing a partial node approach which allows for an extrapolation to the exact answer and developing a variant of release node. Additionally, we find a way to work directly in the thermodynamic limit for some subset of important systems. We apply these new approaches to the Fermi-Polaron finding that it accurately reproduces known analytic and numerical results going beyond them in certain cases [2].

This work was supported in part by ARO Award W911NF-07-1-0464 with funds from the DARPA OLE Program.

[1] G.H. Booth, A. J. W. Thom, and A. Alavi, J. Chem. Phys 131, 5 (2009). [2] M. Kolodrubetz and B.K. Clark, arxiv:1204.1490

Contact: [email protected]

16 2012 Electronic Structure Workshop Wake Forest University

Size-Extensive Wave Functions for Quantum Monte Carlo: A Linear Scaling Generalized Valence Bond Approach

C. Filippi,1 F. Fracchia,2 and C. Amovilli2

1MESA + Institute for Nanotechnology, University of Twente, The Netherlands. 2Dipartimento di Chimica e Chimica Industriale, University of Pisa, Italy.

We present a new class of multideterminantal Jastrow-Slater wave functions constructed with localized orbitals to describe complex potential energy surfaces of molecular systems in quantum Monte Carlo (QMC). We elaborate a coupling scheme between electron pairs, which exploits the local nature of the orbitals and progressively includes new classes of excitations, yielding a sequence of wave functions of increasing complexity. The resulting wave functions are compact, scale linearly with the size of the system, can correlate all valence electrons, and are size extensive. We assess the performance of our wave functions in QMC calculations of the homolytic fragmentation of N-N, N-O, C-O, and C-N bonds, very common in molecules of biological interest. We find excellent agreement with experiments and, even with the simplest forms of our wave functions, we satisfy chemical accuracy and obtain dissociation energies superior to the CCSD(T) results computed with the large cc- pV5Z basis set. We will also present preliminary results on the performance of our QMC wave functions for the estimate of barriers heights of reactions.

This work is funded by HPC-Europa2.

[1] F. Fracchia, C. Filippi, and C. Amovilli, J. Chem. Theory Comput. (2012).

Contact: cfi[email protected]

17 2012 Electronic Structure Workshop Wake Forest University

The Adiabatic-Connection Dissipation-Fluctuation Theorem as Route to a New Generation of Density-Functional Methods

A. G¨orling, A. Heßelmann, and P. Bleizi↵er

Department of Chemistry and Pharmacy, University of Erlangen-N¨urnberg, Germany.

The adiabatic-connection dissipation-fluctuation theorem provides a a rigorous basis for constructing correlation functionals for density-functional methods within the Kohn-Sham formalism [1]. In the simplest case the correlation energy within the direct random phase approximation is obtained. By considering besides the Coulomb kernel also the exact frequency-dependent Kohn-Sham exchange kernel in the construction of the correlation functional, methods with a wide applicability and an unprecedented accuracy are obtained [1–4]. Systems that are problematic or not accessible for standard density-functional meth- ods can be handled. For example, electronic structures characterized by static correlation can be treated [2] as well as Van-der-Waals interactions [3]. On the other hand, chal- lenges like higher computational e↵ort or questions of numerical stability come along with these new approaches. The current status and the perspectives of approaches based on the adiabatic-connection dissipation-fluctuation theorem are discussed.

[1] A. Heßelmann and A. G¨orling, Mol. Phys. 109, 2473 (2011). [2] A. Heßelmann and A. G¨orling, Phys. Rev. Lett. 106, 093001 (2011). [3] P. Bleizi↵er, A. Heßelmann, and A. G¨orling, J. Chem. Phys. 136, 134102 (2012). [4] A. Heßelmann and A. G¨orling, Mol. Phys. 108, 359 (2010).

Contact: [email protected]

18 2012 Electronic Structure Workshop Wake Forest University

Progress and Challenges with Luttinger-Ward Approaches for Going Beyond DFT

S. Ismail-Beigi

Department of Applied Physics and Physics, and Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, CT 06520, USA.

The well-known and well-used Density Functional Theory (DFT) is a variational theory for the ground-state. There are no guarantees that it delivers good results for excited state properties such as quasiparticle energies or wave function. All that is guaranteed is that the total energy is exact, the chemical potential is correct, and that the e↵ective electronic states (Kohn-Sham states) generate the right electron density at the variational minimum. Luttinger-Ward approaches to electronic structure are attractive as beyond-DFT schemes since, in principle, they o↵er a variational functional of the one-electron Green’s function that yields both the correct total energy and all one-particle properties at the variational extremum. However, compared to DFT, these methods face a number of additional chal- lenges beyond the familiar problem of choosing an exchange-correlation functional: (a) they have a much greater computational cost compared to DFT, (b) one must choose some set of trial Green’s functions, and (c) one must locate an extremum (not necessarily a minimum). This talk aims both to be pedagogical and to provide a summary of our most recent work in this area. We focus mainly on the random-phase approximation (RPA) for the correlation functional which yields the GW self-energy for the Green’s function. We will describe ecient approaches for computing the RPA correlation energy; how various types of approximations to the correlation energy create a ladder of approximate self-energy op- erators and what type of physics is included in these approximations (exchange, screening, Hubbard U, etc.); and how the extremization is in fact highly problematic. We will end with some ideas for making practical progress with the extremization and their relation to the successful Quasiparticle Self-consistent GW (QSGW) scheme [1].

Primary support provided by NSF through MRSEC DMR 1119826 (CRISP).

[1] M. van Schilfgaarde, T. Kotani, and S. Faleev, Phys. Rev. Lett. 96, 226402 (2006); Phys. Rev. B 76, 165106 (2007).

Contact: [email protected]

19 2012 Electronic Structure Workshop Wake Forest University

Electrons on a Leash: Topology-Based Charge Partitioning and Rigorous Definition of Oxidation States in Solids

L. Jiang, S. V. Levchenko, and A. M. Rappe

The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA.

The concept of oxidation states (OS) is widely used in all disciplines of physical sciences to explain and predict properties and phenomenons. Its definition stems from a simple ionic charge transfer mechanism and lacks justification in systems with covalent or metallic char- acters. In fact it has been shown that in some transition metal complexes charge transfer does not occur upon changing the OS [1]. In the theoretical front, there are more versatile and accurate definitions of OS, such as charge integration in real-space, and wavefunction projection onto atomic orbitals. However, these traditional methods all have intrinsic am- biguity (e. g. the choice of integration boundaries and atomic basis sets, respectively) and also usually result in non-integer quantities. We propose a method that definitively parti- tions integer number of electrons to a certain nucleus in solid state insulators based solely on the topology of the electronic wavefunctions [2]. When an atomic sublattice (an atom and all of its periodic images) is moved by a lattice vector R~ along a path that keeps the system insulating, the polarization change P~ can be obtained by calculating the Berry phase polarization P~ of each intermediate structure [3]. Such cyclic evolution of the Hamil- tonian produces a quantized P~ that characterizes the charge carried by the sublattice [4]. V P~ R~ The integer quantity N = · , where V is the volume of the unit cell, only depends on e R~ 2 the atomic sublattice in its initial configuration and is therefore defined as the OS of that sublattice. In its essence, the sublattice movement identifies the number of Wannier state centers that move with the sublattice, therefore the belong to the nucleus. As such, each electron can be considered as on an invisible “leash” attached to a certain nucleus and the OS can be uniquely assigned with mathematical rigor. We present results from a broad range of test systems to show that our method not only agrees with conventional chemistry knowledge, but also possesses the ability to resolve real-world ambiguity where experiment falls short.

This work is funded by the AFOSR, DOE BES and ONR.

[1] H. Raebiger, S. Lany, and A. Zunder, Nature 453, 763 (2008). [2] L. Jiang, S. V. Levchenko, and A. M. Rappe, Phys. Rev. Lett. 108, 166 403 (2012). [3] R. D. King-Smith and D. Vanderbilt, Phys. Rev. B. 47, 1651 (1993). [4] J. B. Pendry and C. H. Hodges, J. Phys. C 17, 1269 (1984).

Contact: [email protected]

20 2012 Electronic Structure Workshop Wake Forest University

Tailoring Crystalline Order in Organic Thin-Film Transistors by Exploring Interactions at Interfaces

O. D. Jurchescu,1 J. W. Ward,1 M. A. Loth,2 R. J. Kline,3 and J. E. Anthony2

1Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA. 2Department of Chemistry, University of Kentucky, Lexington, KY 40506, USA. 3Polymers, Semiconductor Electronics, and Surface and Microanalysis Science Division, NIST, Gaithersburg, MD 20899, USA.

Electrical properties of organic crystalline films are determined by their nano- and micro- structure, with a high degree of order, short intermolecular distances and parallel planar molecular orientations encouraging a good overlap between the ⇡-orbitals and a superior charge carrier mobility. However, achieving long-range order in the active semiconducting layers of organic electronic devices is challenging. Film formation using solution deposition methods is a complex process, determined by many processing factors which can a↵ect molecular orientation. We show that the microstructure in organic thin-film transistors can be controlled by manipulating halogen-halogen interactions between the organic semicon- ductor and the self-assembled monolayers (SAMs) present at contact and dielectric surfaces. We selectively choose the surface treatments to introduce targeted interactions during de- position of the organic films, and to isolate key e↵ects behind microstructure formation. We demonstrate that the halogen interactions can template and drive the self-assembly of the organic semiconductor molecules along specific growth fronts and the strength of this inter- action governs molecular alignment within the organic film. As a result, we can precisely control the preferential molecular orientation on surfaces and tune the charge carrier mo- 3 2 2 bility from 10 cm /Vs to 1 cm /Vs in the same material. The e↵ect of SAM presence on thin-film transistor properties such as mobility, contact resistance, interfacial trap-density and threshold voltage will be discussed, and the results will be correlated with structural data obtained from X-ray di↵raction studies.

This work was supported by the National Science Foundation (ECCS-1102275).

Contact: [email protected]

21 2012 Electronic Structure Workshop Wake Forest University

Electronic Structure of Dye-Sensitized TiO2 Clusters from G0W0

N. Marom

Center for Computational Materials, Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, TX 78712, USA.

The development of new types of solar cells is driven by the need for clean and sustainable energy. In this respect dye sensitized solar cells (DSC) are considered as a promising route for departing from the traditional solid state cells. The physical insight provided by compu- tational modeling may help develop improved DSCs. To this end it is important to obtain an accurate description of the electronic structure, including the fundamental gaps and level alignment at the dye-TiO2 interface. This requires a treatment beyond ground-state density functional theory (DFT). In this talk I will present a many-body perturbation theory study, within the G0W0 approximation, of TiO2 clusters, dye molecules, and dye-sensitized TiO2 clusters. G0W0 calculations, based on a hybrid functional, are in excellent agreement with photoemission spectroscopy (PES) experiments. For TiO2 clusters, this enables the identifi- cation of the isomers observed in PES as those with the highest vertical electron anity [1]. For transition metal phthalocyanine dyes, unlike DFT, G0W0 calculations correctly predict the energies of orbitals associated with the metal d-states [2]. For dye-sensitized TiO2 clus- ters, G0W0 calculations provide more reliable predictions than DFT for the fundamental gaps and the level alignment at the dye-TiO2 interface [3].

This work was funded by NSF grants DMR-0941645 and OCI-1047997, and DOE grant DE-SC0001878. Computational resources were provided by NERSC and TACC.

[1] N. Marom, M Kim, and J. R. Chelikowsky, Phys. Rev. Lett. 108, 106801 (2012). [2] N. Marom, X. Ren, J. E. Moussa, J. R. Chelikowsky, and L. Kronik, Phys. Rev. B 84, 195143 (2011). [3] N. Marom, J. E. Moussa, X. Ren, A. Tkatchenko, and J. R. Chelikowsky, Phys. Rev. B 84, 245115 (2011).

Contact: [email protected]

22 2012 Electronic Structure Workshop Wake Forest University

Density-Functional Theory: Time to Move on?

N. Marzari

Theory and Simulation of Materials, THEOS IMX STI EPFL, Lausanne CH-1015, Switzerland.

Density-functional theory can be a very powerful tool for scientific discovery and techno- logical advancement. Still, it remains an imperfect tool, with open and urgent challenges in our quest towards qualitative and quantitative accuracy, and in our ability to perform quantum simulations under realistic conditions. Several of these challenges stem from the remnants of self-interaction in our electronic- structure framework, leading to qualitative failures in describing some of the fundamental processes involved in energy applications - from charge-transfer excitations to photoemis- sion spectra, to the structure and reactivity of transition-metal complexes. Ill discuss these challenges in realistic case studies, and our suggestions for possible solutions - including constrained DFT, DFT + onsite and intersite Hubbard terms, and Koopmans’ compli- ant energy functionals. In particular, I’ll discuss how self-interaction corrected functionals lead naturally to a beyond-DFT formulation where both total energies and spectroscopic properties can be accounted for.

Contact: nicola.marzari@epfl.ch

23 2012 Electronic Structure Workshop Wake Forest University

On the Molecular Dissociation of Dense Hydrogen and the Solid/Liquid Transition in the Atomic Phase via Free-energy Calculations

J. M. McMahon

The Institute for Condensed Matter Theory, Department of Physics, University of Illinois at Urbana-Champaign Urbana, IL 61801-3080, USA.

In 1935, Wigner and Huntington predicted that sucient pressure would cause hydrogen to become atomic and metallic. Since then, theoretical predictions have suggested that the atomic phase will exhibit remarkable properties, perhaps most notably possible quantum melting at low- or zero- temperature. Simulations to support these predictions, however, are extremely challenging, as they must consider numerous complex e↵ects that could drive such a transition, including nuclear quantum e↵ects and entropic (de-)stabilization at finite temperature. In this talk, accelerated path-integral molecular dynamics simulations of the solid and liquid atomic phases are performed at pressures near molecular dissociation to the ultrahigh regime. Coupling-constant integrations at finite temperature are performed to estimate the entropies of the two phases at reference points, and thermodynamic integrations are used to determine the Gibbs free-energies over a wide range of pressure and temperature conditions. From these results, we construct the phase diagram of dense hydrogen, from molecular dissociation to ultrahigh pressures.

Contact: [email protected]

24 2012 Electronic Structure Workshop Wake Forest University

Dirac Semimetal in Three Dimensions

A. M. Rappe,1 S. M. Young,1 S. Zaheer,2 J. C. Y. Teoa,2 C. L. Kane,2 and E. J. Mele2

1The Makineni Theoretical Laboratories Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA. 2Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104-6396, USA.

In a Dirac semimetal, the conduction and valence bands contact only at discrete (Dirac) points in the Brillouin zone (BZ) and disperse linearly in all directions around these critical points. Including spin, the low energy e↵ective theory around each critical point is a four band Dirac Hamiltonian. In two dimensions (2D), this situation is realized in graphene without spin-orbit coupling [1]. 3D Dirac points are predicted to exist at the phase transition between a topological and a normal insulator in the presence of inversion symmetry [2, 3]. Here we show that 3D Dirac points can also be protected by crystallographic symmetries in particular space-groups and enumerate the criteria necessary to identify these groups. This reveals the possibility of 3D analogs to graphene. We provide a systematic approach for identifying such materials and present ab initio calculations of metastable -cristobalite BiO2 which exhibits Dirac points at the three symmetry related X points of the BZ.

[1] C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005). [2] S. Murakami, New J. Phys. 9, 356 (2007). [3] S. M. Young et al., Phys. Rev. B 84, 085106 (2011).

Contact: [email protected]

25 2012 Electronic Structure Workshop Wake Forest University

Topological Order in Electronic Wavefunctions

R. Resta

Dipartimento di Fisica, Universit`adi Trieste, Trieste, Italy.

The geometrical and topological properties of electronic wave functions are commonly ad- dressed in terms of the Berry connection and Berry curvature, both defined in some param- eter space. The integral of the curvature over a closed surface equals 2⇡ times an integer C Z, called the (first) Chern number. C is measurable, in principle with infinite accu- 2 racy: 109 has been attained for the quantum Hall e↵ect. Di↵erent values of C characterize topologically distinct electronic states; C is a topological invariant, extremely robust under changes in material parameters. The corresponding physical property is “topologically pro- tected”: in insulators the C value can be switched only passing through a metallic state. A C = 0 value requires absence of time-reversal symmetry. 6 For a crystalline insulator of noninteracting electrons the relevant parameter is the Bloch vector k, the Berry curvature is the imaginary part of the Marzari-Vanderbilt metric, and the closed surface is the Brillouin zone (a torus). The paradigmatic systems are two- dimensional. I will illustrate two nonconventional approaches which can also cope with noncrystalline systems (disordered or macroscopically inhomogeneous). I will show how, in both approaches, C is evaluated avoiding any k-integration. The rationale is that the topological order measures the entanglement of the ground state in the bulk of the sample. The k vector is an artificial construct: all ground state properties-including topological order–are embedded in the ground state density matrix, “shortsighted” in insulators. The first approach [1] adopts periodic boundary conditions in a supercell framework, where the ground state is obtained from a single Hamiltonian diagonalization at k = 0. The Berry curvature is defined as a response of the electronic wave function to an infinitesimal k change. For a large enough supercell, the value of the curvature at k = 0 is enough to yield C : the major result is that it is computed with no extra diagonalization. The second approach [2] introduces a topological marker, defined in r space, and which may vary in di↵erent regions of the same sample. Notably, this applies equally well to periodic and open boundary conditions. Simulations over a model Hamiltonian validate our theory. Our test cases include crystalline as well as disordered samples, and heterojunctions. While the search for C 0 materials in zero B field is controversial, new classes of time- 2 reversal symmetric topological insulators were predicted and discovered recently. Therein a novel two-valued invariant (Z2) discriminates topological order. Even the Z2 invariant can be defined and computed via integrals of the connection and curvature in textbfk space, while no approach is available to cope with inhomogeneous systems (e.g. heterojunctions).

[1] D. Ceresoli and R. Resta, Phys. Rev. B 76, 012405 (2007). [2] R. Bianco and R. Resta, Phys. Rev. B 84, 241106 (R) (2011).

Contact: [email protected]

26 2012 Electronic Structure Workshop Wake Forest University

A Generalized Solid-State Nudged Elastic Band Method

D. Sheppard,1,2 P. Xiao,2 W. Chemelewski,2 D. D. Johnson,3 and G. Henkelman2

1Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 2Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78712, USA. 3Ames Laboratory, DOE, TASF 111, Ames, IA 50011, USA. Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA.

I will present a generalized solid-state nudged elastic band (G-SSNEB) method for deter- mining reaction pathways of solid-solid transformations involving both atomic and unit- cell degrees of freedom. This work combines Henkelman el al.’s atomistic NEB [1] with Caspersen and Carter’s periodic cell based NEB [2] in an e↵ort to avoid dragging coordi- nates which can result in a bad reaction coordinate and inaccurate saddle point energies. Atomic and cell degrees of freedom are combined into a unified description of the crystal structure so that calculated reaction paths are insensitive to the choice of periodic cell. For the rock-salt–to–Wurtzite transition in CdSe, I demonstrate the method is robust for mechanisms dominated either by atomic motion or by unit-cell deformation; notably, the lowest-energy transition mechanism found by the G-SSNEB changes with cell size from a concerted transformation of the cell coordinates in small cells to a nucleation event in large cells. The method is ecient and can be applied to systems in which the force and stress tensor are calculated using density functional theory.

The work was supported by the National Science Foundation (CHE-0645497, DMR-07-05089) and the Robert A. Welch Foundation (F-1601). Additional funding came from Department of Energy, Basic Energy Sciences, Division of Materials Science and Engineering (DEFG02-03ER46026), Ames Laboratory (DE-AC02-07CH11358), which is operated by Iowa State University, and Los Alamos National Laboratory (DE-AC52-06NA25396), operated by Los Alamos National Security, LLC, for the NNSA. Computing was provided by the Texas Advanced Computing Center.

[1] G. Henkelman, B. P. Uberuaga, and H. J´onsson, J. Chem. Phys. 113, 9901 (2000). [2] K. J. Caspersen and E. A. Carter, Proc. Nat. Acad. Sci. 102, 6738 (2005).

Contact: [email protected]

27 2012 Electronic Structure Workshop Wake Forest University

First-Principles Calculation of Topological Invariants

A. A. Soluyanov and D. Vanderbilt

Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA.

The discovery of topological insulators opened a new direction in band theory. The topology of a time-reversal symmetric insulating bulk band structure was shown to have a one-to-one correspondence to the presence of robust metallic edge or surface states in 2D (quantum spin Hall) or 3D (strong topological) insulators. We address the question of how to compute the topological indices of time-reversal symmetric insulators by means of a standard band structure calculation. In particular, we use an analysis based on hybrid Wannier functions, i.e., states that are localized in one direction in real space but remain Bloch-like in the other directions. Tracking the positions of the charge centers of these functions as a function of k allows one to obtain the topological invariants of the band structure in question [1]. The method is discussed from the point of its actual implementation in ab initio codes.

This work is funded by the NSF.

[1] A. A. Soluyanov and D. Vanderbilt, Phys. Rev. B 83, 235401 (2011).

Contact: [email protected]

28 2012 Electronic Structure Workshop Wake Forest University

Dense Hydrogen by First-Principles Path-Integral Molecular Dynamics: Structure of Phase II, Melting Curve, and Beyond

M. Torrent, G. Geneste, F. Bottin, and P. Loubeyre

CEA, DAM, DIF, F-91297 Arpajon, France.

The role of quantum nuclear zero-point motion in the structural changes of solids H2 and D2 under pressure is investigated for the low-temperature phases and around the melting curve by first-principles path-integral molecular dynamics (PIMD) calculations [1]. A radical revision of the current interpretation of the nature of phase II is presented. An isotope di↵erence for the structure of phase II is discovered. Phase II of solid D2 is identical to the classical counterpart (C2/c space group). Phase II of solid H2, exhibits large and very asymmetric angular quantum fluctuations. The quantum-mechanically averaged structure (close to Cmcm), is di↵erent from the most probable one (Cmc21). That fits the concept of ”quantum fluxional solid”, which cannot be understood in terms of a single classical structure. The mechanism of the transition into phase III is also obtained. Existing structural data support this microscopic interpretation. The e↵ect of quantum fluctuations of nuclei on the melting curve and the Plasma Phase Transition is also investigated. We have used the ABINIT code [2], in which we have implemented the path-integral formalism for nuclei. For the sake of numerical eciency, we have introduced in ABINIT, beyond the already existing levels of parallelization, an additional level of parallelization on the system replicas associated to the discretization of the PI in imaginary time. This level of parallelization has a quasi-linear scalability and, combined to the others, allowed us to perform, by using several thousands of CPU cores, very long DFT-PIMD trajectories of at least 30 000 steps, and up to 100 000 steps in some cases where a high level of statistics was required.

[1] G. Geneste, M. Torrent, F. Bottin, and P. Loubeyre, http://arxiv.org/abs/1205.2290. [2] X. Gonze et al., Computer Phys. Comm. 180, 2582 (2009).

Contact: [email protected]

29 2012 Electronic Structure Workshop Wake Forest University

Adaptive Genetic Algorithm Method for Crystal Structure Prediction and Materials Discovery

C. Z. Wang

Ames Laboratory USDOE and Department of Physics, Iowa State University, Ames, IA 50011, USA.

The urgent demand for new energy technologies has put great pressure on the capabili- ties of today’s materials and chemical research. Accurate and fast computational struc- ture/property determinations can complement the traditional experimental try and error e↵orts in material design and accelerate the pace of technological advances. We have re- cently developed a fast and ecient method for crystal structure prediction and materials discovery. Our method performs genetic algorithm (GA) searches using auxiliary classical potentials to screen the energies of candidate structures, and select only a few of them for more extensive first principles evaluation. Parameters of the auxiliary potentials are adap- tively adjusted to reproduce the first-principles results during the course of the GA search. Therefore, the adaptive GA method can have the speed of empirical potential searches but with the accuracy of first principles calculations. The eciency of the adaptive GA method allows a great increase in the size and complexity of systems that can be studied. We will present results on applications to various systems including metallic alloys and ultrahigh pressure SiO2,H2O and Mg-Si-O systems.

Work supported by US DOE-BES and in collaboration with Min Ji, Shunqing Wu, Manh Cuong Nguyen, Koichiro Umemoto, Renata M. Wentzcovitch, and Kai-Ming Ho.

Contact: [email protected]

30 2012 Electronic Structure Workshop Wake Forest University

Spin Crossover Systems in the Deep Mantle

R. M. Wentzcovitch

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA.

There has been much interest in spin crossovers found experimentally in the most abun- dant minerals of Earths lower mantle ((MgFe)O, (MgFe)(Si,Fe)O3-perovskite and post- perovskite) under pressure. Spin crossovers depend strongly on thermodynamic conditions and a full understanding of this problem requires its investigation as function of pressure and temperature. Several aspects of this phenomenon have been dicult to understand based on experiments alone, especially in the perovskite system, and electronic structure calculations are helping to clarify them. The geophysical consequences of these crossovers are yet to be fully understood. I will review progress we have made in understanding spin crossovers and give an overview of this phenomenon and its potential implications for the Earth.

Research in collaboration with H. Hsu, K. Umemoto, P. Blaha, J. F. Justo, S. de Gironcoli, C. R. S. da Silva, and T. Tsuchiya. Research supported by NSF/ATM-0428774, EAR-0810212, and EAR-1047629, and in part by the MRSEC Program of NSF under Award Number DMR-0212302 and DMR-0819885.

Contact: [email protected]

31 2012 Electronic Structure Workshop Wake Forest University

Auxiliary-Field Quantum Monte Carlo Calculations of Excited States and Strongly Correlated Systems

S. Zhang

Department of Physics, College of William & Mary, Williamsburg, VA 23185, USA.

I will describe our recent e↵orts to develop quantum Monte Carlo (QMC) approaches for excited states and for more systematic and accurate treatment of ground states of strongly correlated sytems and heavier elements. We use the auxiliary-field quantum Monte Carlo (AFQMC) framework, which is a many-body total energy method that looks like many coupled mean-field calculations. I will discuss this method’s relations and di↵erences with traditional density-functional and quantum chemistry methods, and with other forms of QMC. The AFQMC method uses importance-sampled random walks in the space of non- orthogonal Slater determinants to project out the many-body ground state, or excited state. The random walks are constrained by a trial wave function (TWF) to remove the so-called sign problem, giving approximate (but non-perturbative) results. With single- determinant TWFs from Hartree-Fock or density-functional theory (DFT), this approach can be thought of as a post-DFT method with no parameter. In this form, the accuracy is typically comparable to the quantum chemistry coupled-cluster CCSD(T) method near equilibrium geometries, and better than CCSD(T) when bonds are stretched. The compu- tational cost of AFQMC scales as N 3 with characteristic size of the system. Its accuracy can be further improved at the cost of increased computing (releasing the constraint) or bet- ter TWF. With AFQMC, we can systematically target selected orbitals to correlate. This allows many-body calculations in solids with a frozen core, with down-folded Hamiltonians, or possibly embeded in a DFT calculations to extend scales. Results from a few on-going applications will be presented, including band structures in simple solids, magnetic phases in MnO, and Cobalt adsoption on graphene.

This work is funded by the NSF and DOE; computing on Jaguar at ORNL. In collaboration with W. Purwanto, F. Ma, Y. Virgus, H. Krakauer, and H. Shi.

Contact: [email protected]

32 2012 Electronic Structure Workshop Wake Forest University

Homogeneous Electric and Magnetic Fields in Periodic Systems

J. W. Zwanziger

Departments of Chemistry and Physics and Atmospheric Sciences, and Institute for Research in Materials, Dalhousie University, Halifax, NS, Canada B3H 4R2.

Because the application of external homogeneous electric or magnetic fields breaks the pe- riodicity of a solid, the correct theoretical treatment of the response to such fields has been achieved remarkably recently. The modern theory of polarization showed how the polariza- tion of an infinite solid could be obtained from consideration of the Bloch wavefunctions in a unit cell [1–3], and likewise recent work on the magnetization has yielded similar results [4, 5]. Both of these results can be understood in terms of geometric phase arguments. In the present talk I will try to accomplish three objectives: to provide an overview of the geometric phase arguments that underpin these results; to discuss our recent extensions of the implementation of the polarization theory in a density-functional theory code to include the projector-augmented wave formalism [6] as well as spin-orbit coupling; and to outline our new theory of magnetic response in insulators and describe the current state of its implementation [7].

Funding from NSERC and the Canada Research Chairs program is acknowledged.

[1] R. Resta, Rev. Mod. Phys. 66, 899 (1994). [2] R. D. King-Smith and D. Vanderbilt, Phys. Rev. B 47, 1651 (1993). [3] R. W. Nunes and X. Gonze, Phys. Rev. B 63, 155107 (2001). [4] D. Ceresoli, T. Thonhauser, D. Vanderbilt, and R. Resta, Phys. Rev. B 74, 024408 (2006). [5] D. Xiao, J. Shi, and Q. Niu, Phys. Rev. Lett. 95, 137204 (2005). [6] J. W. Zwanziger, J. Galbraith, Y. Kipouros, M. Torrent, M. Giantomassi, and X. Gonze, Comp. Mater. Sci. 58, 113 (2012). [7] X. Gonze and J. W. Zwanziger, Phys. Rev. B 84, 064445 (2011).

Contact: [email protected]

33 2012 Electronic Structure Workshop Wake Forest University

34 2012 Electronic Structure Workshop Wake Forest University

3.2 Abstracts of Posters

Abstracts are sorted alphabetically by last name of the presenting author. If an abstract has several authors, the presenting author is listed first. An index of posters can be found in Section 5.2 on page 114. For the poster sessions, the poster number is determined by the page number of the corresponding abstract. Even numbered posters are presented in Poster Session A on Wednesday, June 6, 2012 at 2:00 pm. Odd numbered posters are presented in Poster Session B on Thursday, June 7, 2012 at 2:00 pm. All poster sessions are in 401 Benson.

35 2012 Electronic Structure Workshop Wake Forest University

Physisorption of Three Amine Terminated Molecules (TMBDA, BDA, TFBDA) on the Au(111) Surface: The Role of van der Waals Interactions

M. Aminpour,1 D. Le,1 A. Kiejna,2 and T. S. Rahman1

1Department of Physics, University of Central Florida, Orlando, FL 32816, USA. 2Institute of Experimental Physics, University of Wroclaw, 50-204 Wroclaw, Poland.

Recently, the electronic properties and alignment of tetramethyl-1,4-benzenediamine (TM- BDA), 1,4-benzenediamine (BDA) and tetrafluro-1,4-benzenediamine (TFBDA), molecules were studied experimentally. Discrepancies were found for both the binding energy and the molecule tilt angle with respect to the surface, when results were compared with den- sity functional theory calculations [1]. We have included the e↵ect of vdW interactions both between the molecules and the Au(111) surface and find binding energies which are in very good agreement with experiments. We also find that at low coverages each of these molecules would adsorb almost parallel to the surface. N-Au bond lengths and charge re- distribution on adsorption of the molecules are also analyzed. Our calculations are based on DFT using vdW-DF exchange correlation functionals. For BDA (since we are aware of experimental data), we show that for higher coverage, inclusion of intermolecular van der Waals interaction leads to tilting of the molecules with respect to the surface and forma- tion of line structures. Our results demonstrate the central role played by intermolecular interaction in pattern formation on this surface.

This work was supported in part by DOE-Grant DE-FG02-07ER15842.

[1] M. Dell’Angela et al., Nano Lett. 10, 2470 (2010). [2] M. Kamenetska et al., J. Phys. Chem. C 115, 12625 (2011).

Contact: [email protected]

36 2012 Electronic Structure Workshop Wake Forest University

E↵ects of Disordered Substitutions and Vacancies in Fe Based Superconductors from First Principles

T. Berlijn,1 L. Wang,1 Y. Wang,2 C.-H. Lin,1,3 W. Garber,1 P. J. Hirschfeld,2 and W. Ku1,3

1Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA. 2Department of Physics, University of Florida, Gainesville, FL 32611, USA. 3Physics Department, State University of New York, Stony Brook, NY 11790, USA.

Using a recently developed Wannier function based first principles method [1], we compute the configuration-averaged spectral function A(k, !) of Fe based superconductors contain- h i ing disordered substitutions and vacancies. In the transition metal doped Ba(Fe1 xMx)2As2 with M=Co/Zn we find in addition to an increased chemical potential a loss of coherent carrier spectral weight [2]. For the case of isovalent substitutions we find the Fermi surface to be protected against the influence of disorder, surprisingly even in Ba(Fe1 xRux)2As2. For the case of disordered Fe and K vacancies in K0.8Fe1.6Se2 we find a Fermi surface con- sisting of large/small electron pockets in the zone corners/center [3], in good agreement with ARPES measurements.

Work supported by DOE DE-AC02-98CH10886 and DOE CMCSN.

[1] T. Berlijn, D. Volja, and W. Ku, Phys. Rev. Lett. 106, 077005 (2011). [2] T. Berlijn, C.-H. Lin, W. Garber, and W. Ku, Phys. Rev. Lett. 108, 207003 (2012). [3] T. Berlijn, P. J. Hirschfeld, and W. Ku, arXiv:1204.2849.

Contact: [email protected]

37 2012 Electronic Structure Workshop Wake Forest University

Structural and Electronic Trends in Rutile Compounds

T. Birol, L. A. Salguero, and C. J. Fennie

School of Applied Engineering Physics, Cornell University, Ithaca, NY 14850, USA.

Rutile structure is ubiquitous in transition metal (TM) dioxides: All but two early TM dioxides at least have an polymorph that has either the rutile structure or a slightly dis- torted variant of it. Ideal, tetragonal rutile structure involves TM ions at the center of almost perfect oxygen octahedra, which form edge sharing chains. A common, slightly dis- torted variant of the rutile structure is the so called MoO2 structure, which involves cations forming pairs along these edge sharing octahedron chains. While there are various studies on individual compounds and why they form in undistorted rutile or the MoO2 structures, there is no single study that explains the structural trends in early TM dioxides. In this work we present a complete study of structural and electronic trends in early TM dioxides. We show how the interplay of a pseudo-Peierls mechanism, proposed as the driving force for the dimerization in compounds like VO2 [1] or MoO2 [2], with structural degrees of freedom lead to rich, nonmonotonic trends in these compounds. This provides a beautiful example of how electronic structure can lead to structural changes in the well known rutile structure.

This work is supported as part of the Energy Materials Center at Cornell (EMC2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Oce of Science, Oce of Basic Energy Sciences under Grant No. DE-SC0001086.

[1] A. S. Belozerov, M. A. Korotin, V. I. Anisimov, and A. I. Poteryaev, Phys. Rev. B 85, 045109 (2012). [2] V. Eyert, R. Horny, K. H. Hock, and S. Horn, J. Phys.: Cond. Mat. 12, 4923 (2000).

Contact: [email protected]

38 2012 Electronic Structure Workshop Wake Forest University

Laplacian-Based Models for the Exchange Energy

A. C. Cancio1 and C. E. Wagner1,2

1Department of Physics and Astronomy, Ball State University, Muncie, IN 47306, USA. 2Currently at Department of Physics, University of Florida, FL, USA.

Recent Quantum Monte Carlo data for the exchange-correlation energy density of pseu- dopotential systems strongly indicate the significant role of the Laplacian of the density in characterizing the first order correction to the local density approximation of density functional theory. We report on an exchange functional built upon these observations and extended to the all-electron case. The model keeps the typical properties of constraint- based generalized gradient approximations (GGA) and also has a finite-valued potential at the nucleus, unlike the GGA. Problems with oscillatory behavior in the potential due to higher order derivatives are controlled by a curvature minimization constraint. The results are tested against exact potentials for the He, Be, and Ne atoms. A combination of gradi- ent and Laplacian as suggested by a gradient expansion of the exchange hole gives the best overall results.

This work is funded by NSF grant DMR-0812195.

Contact: [email protected]

39 2012 Electronic Structure Workshop Wake Forest University

Quasiparticle Band Structure Calculation of Monolayer, Bilayer, and Bulk MoS2

T. Cheiwchanchamnangij and W. R. L. Lambrecht

Department of Physics, Case Western Reserve University, Cleveland, OH 44106, USA.

Quasiparticle self-consistent GW calculations of the band structures and related e↵ective mass parameters are carried out for bulk, monolayer and bilayer MoS2. Including excitonic e↵ects within the Mott-Wannier theory, quantitative agreement is obtained between the A, B excitons, measured by absorption [1], and the calculated exciton gap energies at K. The A-B splitting arises from the valence band splitting which in the monolayer is entirely due to spin-orbit coupling and leads to spin-split states, while in the bilayer it is a combined e↵ect of interlayer and spin-orbit coupling.

This work was supported by the National Science Foundation under grant number DMR-1104595 and made use of the High Performance Computing Resource in the Core Facility for Advanced Research Computing at Case Western Reserve University.

[1] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Phys. Rev. Lett. 105, 136805 (2010).

Contact: [email protected]

40 2012 Electronic Structure Workshop Wake Forest University

Spectral Propagation Schemes for TDDFT Calculations with Applications to Molecules and CNTs

Z. Chen and E. Polizzi Department of Electrical and Computer Engineering, University of Massachusetts at Amherst, Amherst, MA 01003, USA.

In nanoelectronic applications, ecient time-dependent simulations have become increas- ingly important for characterizing the electron dynamics under time dependent external perturbations such as electromagnetic fields, pulsed lasers, and particle scattering. Reli- able modeling approaches in time domain, however, are often limited in term of trade-o↵ between robustness and performances, and a direct numerical treatment is dicult. In this work, we aim to go beyond these limitations by introducing high-performance numerical propagation schemes to compute the solution of the time-ordered evolution op- erator. The numerical treatment of these evolution operators often gives rise to the matrix exponential, commonly treated using approximations such as split-operator techniques. In contrast, the eciency of the time-domain propagation techniques described here, is en- hanced by reliance on the capabilities of the FEAST algorithm for solving the eigenvalue problem [1]. Using FEAST the eigenvalue problem is reformulated into solving a set of well- defined independent linear systems along a complex energy contour. Obtaining the spectral decomposition of the matrix exponential (i.e. direct diagonalization of the Hamiltonian) be- comes then a suitable alternative to PDE based techniques such as Crank-Nicolson schemes, since larger time-intervals can now be considered and all the resulting linear systems can be solved in parallel. Additionally, FEAST has the ability to re-use the basis of a precom- puted subspace as suitable initial guess for solving the series of eigenvalue problems that are close one another all along the time-domain propagation. In order to reduce even fur- ther the number of large-scale eigenvalue problems to be computed, two highly optimized propagation schemes (Gauss and BTPS) [2] have also been proposed and implemented. Within RT-TDDFT (ALDA) framework, our model consider all-electron calculations, real-mesh techniques (finite-element method), and real time propagation of the Kohn-Sham orbitals. In order to obtain the optical responses, we apply a weak delta kick to the system, let it evolve over time, then Fourier transform the dipole moment to get the optical oscillator strength. The spectroscopy results shows very good agreement with the experiments for various molecules ranging from H2 to C60. The time-dependent framework has also been used to obtain the THz response of metallic (5,5) CNT (isolated) devices and reproduce the experimental results of the Fermi-velocity and kinetic inductance [3]. [1] E. Polizzi, Phys. Rev. B 79, 115112 (2009). [2] Z. Chen and E. Polizzi, Phys. Rev. B 82, 205410 (2010). [3] Z. Chen, S. Yngvesson, and E. Polizzi, 11th IEEE-NANO 2011, Conf. Proceed. 1339 (2011).

Contact: [email protected]

41 2012 Electronic Structure Workshop Wake Forest University

Intrinsic Voltage Limit and Crystal Field Splitting in Silicate Cathode Materials for Li-Ion Batteries

K. Cho, R. C. Longo, K. Xiong, and K. C. Santosh

Materials Science and Engineering Department, The University of Texas at Dallas, Richardson, TX 75080, USA.

The current needs to overcome our dependence on the fossil fuels have drawn a grow- ing interest to the development of more advanced energy storage systems. Among them, rechargeable Li-ion batteries have been the focus of numerous experimental and theoreti- cal studies. The two key properties to characterize the energy density of a Li-ion battery electrode are the specific capacity and voltage of operation. While the specific capacity theoretical limit is given by the chemical formula of the di↵erent compounds, the voltage depends on the morphology, relative stability and other electronic properties of the electrode material. In our study, we have focused our attention in polyoxyanion silicates as positive electrode materials. These materials incorporate transition metals (TM) in the host struc- tures to allow the necessary electronic conductivity for the open circuit of the battery, and their voltage is mainly due to the redox activity of these TM. There is always an intrinsic [1] voltage limit for every TM redox couple, and it depends on the position of their elec- tronic d-bands and the hybridization with the anion (oxygen) electronic p-states. In this work, using density-functional theory (DFT) methods, we perform a detailed analysis of the electronic structure of two di↵erent silicate morphologies with di↵erent redox couples, to find out which is the influence of the crystal structure on the electronic d-bands of the silicate and, by comparing our results with those of regular oxides, to propose mechanisms to control the position of the electronic d-bands and, thus, the voltage and energy density of the Li-ion battery.

The authors acknowledge the Texas Advanced Computing System (TACC) for the computational resources provided.

[1] J. B. Goodenough and Y. Kim, Chem. Mater. 22, 587 (2010).

Contact: [email protected]

42 2012 Electronic Structure Workshop Wake Forest University

First Principles Investigation of Structural and Electronic Properties of Hexagonal Rare-Earth Ferrites, RFeO3 (R = Lu-Tm)

H. Das and C. J. Fennie

School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA.

The multiferroic hexagonal manganites RMnO3 (R=Dy-Lu,Y) belong to a fascinating class of materials that display an unusual, complex interplay between structural, polar and mag- netic domains [1, 2]. The electric polarization in these compounds is found to be a by- product of a trimerized (zone-boundary) lattice distortion, arising from the ionic size mis- match between R+3 and Mn+3 ions [3, 4]. Both theoretical [4] and experimental [5, 6] studies suggest that an immediate transition from the high temperature paraelectric phase (P63/mmc) to the trimerized polar phase (P63cm) takes place in these compounds. The hexagonal ferrites RFeO3 (R=Er-Lu,Y) crystallize in the same polar structure as the manganite counterparts. However, in case of YbFeO3, one of the members of the hexagonal ferrite systems, the P63/mmc to P63cm transition is reported to be mediated by an intermediate polar phase (P63mc) [7]. This indicates a di↵erent underlying energetics of the ferrite systems compare to the manganite counterparts, which is verified in the present study through first principles density functional calculations.

This work is funded by the DOE-BES- SISGR and Penn State, NSF MRSEC.

[1] T. Choi, Y. Horibe, H. T. Yi, Y. J. Choi, W. Wu, and S.-W. Cheong, Nature Mater. 9, 253 (2010). [2] T. Jungk, A. Ho↵mann, M. Fiebig, and E. Soergel, Appl. Phys. Lett. 97, 012904 (2010). [3] B. B. Van Aken, T. T. M. Palstra, A. Filippetti, and N. A. Spaldin, Nature Mater. 3, 164170 (2004). [4] C. J. Fennie and K. M. Rabe, Phys. Rev B 72, 100103(R) (2005). [5] I.-K. Jeong, N. Hur, and T. Pro↵en, J. Appl. Crystall 40, 730 (2007). [6] A. S. Gibbs, K. S. Knight, and P. Lightfoot, Phys. Rev. B 83, 094111 (2011). [7] Y. K. Jeong, J.-H. Lee, S.-J. Ahn, S.-W. Song, H. M. Jang, H. Choi, and J. F. Scott, J. Am. Chem. Soc. 134, 1450 (2012).

Contact: [email protected]

43 2012 Electronic Structure Workshop Wake Forest University

Ab Inito-Based Interatomic Potentials for Body-Centered Cubic Refractory Metals

M. R. Fellinger, H. Park, J. W. Nicklas, and J. W. Wilkins

Department of Physics, The Ohio State University, Columbus, OH 43210, USA.

A fundamental understanding of transformation and deformation processes in the bcc re- fractory metals (V, Nb, Ta, Mo, and W) is vital for designing new bcc-based commercial alloys with desired properties. Such an understanding is aided by computational methods capable of reaching length and time scales needed for meaningful simulations of phase trans- formations and extended defects responsible for plastic deformation. Classical interatomic potentials are indispensable for simulating such phenomena inaccessible to first-principles methods. We develop accurate and robust modified embedded-atom method (MEAM) po- tentials [1, 2] for the bcc metals by fitting the model parameters to accurate first-principles data. The potentials are applicable for studying mechanical and thermodynamic properties, yielding excellent agreement with both experiments and first-principles calculations.

Supported by DOE-Basic Energy Sciences, Division of Materials Sciences (DE-FG02-99ER45795). Computational resources provided by OSC and NERSC.

[1] M. I. Baskes, Phys. Rev. Lett. 59, 2666 (1987). [2] T. J. Lenosky, B. Sadigh, E. Alonso, V. V. Bulatov, T. D. de la Rubia, J. Kim, A. F. Voter, and J. D. Kress, Model. Simul. Mater. Sci. Eng. 8, 825 (2000).

Contact: [email protected]

44 2012 Electronic Structure Workshop Wake Forest University

Modeling Piezoelectricity in Improper Ferroelectrics

K. F. Garrity and K. M. Rabe

Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA.

Piezoelectric materials are key components of many important technologies, and discover- ing materials with improved piezoelectric responses is a major goal of materials science. In particular, finding new mechanisms for piezoelectricity which allow for high piezoelectric coecients, especially in lead-free materials, could have great technological impact. While piezoelectric coecients can be calculated directly with standard electronic structure codes, accurate modeling of the contributions to piezoelectric responses, and in particular the com- plicated interplay between atomic relaxations and strain, is needed in order to understand and explore new mechanisms for piezoelectric response. In this work, we present our model for the piezoelectric response in Ca3Ti2O7, an improper ferroelectric which we find has large piezoelectric coecients. We expand the energy in terms of strain and a small number of important atomic displacements, and then minimize over the remaining degrees of freedom, allowing our model to accurately capture the piezoelectric response with a minimal number of degrees of freedom.

Contact: [email protected]

45 2012 Electronic Structure Workshop Wake Forest University

A Nonlinear Eigensolver-Based Alternative to Traditional SCF Methods

B. Gavin and E. Polizzi

Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA 01003, USA.

We present an iterative solution for nonlinear eigenvector problems (i.e. H( ) = E ) { } such as the one arising from the Kohn-Sham equations in density functional theory (DFT). The new strategy o↵ers a very ecienct and robust alternative to the traditional self- consistent field (SCF) procedures used in electronic structure calculations (such as Newton- Broyden, Anderson mixing, direct inversion of the iterative subspace and mixed Pulay techniques). This is accomplished using a modification of the FEAST algorithm [1] applied to the full non-linear eigenvalue problem. From the complex contour integration of the projected Green’s function, it is possible to obtain an approximate eigenvector subspace which is then used to derive a reduced non-linear eigenvector problem that is orders of magnitude smaller than the original one. The search subspace is then updated at each iteration of the FEAST algorithm until some quantity of interest (e.g. non-linear eigen- vector residuals or total energy) converges (convergence is typically obtained in less than 10 iterations). The resulting approach is potentially one order of magnitude faster than conventional iterative methods, and it converges to the correct solution regardless of the choice of initial guess. Numerical examples are provided for various molecules (from H2 to C60) using Kohn-Sham/DFT/LDA and an all-electron finite element numerical frame- work in order to illustrate the robustness and eciency of the new approach. A practical and straightforward ‘black-box’ implementation of the numerical strategy using the FEAST library package [2] will also be provided.

This work is funded by Intel the NSF #0846457.

[1] E. Polizzi, Phys. Rev. B 79, 115112 (2009). [2] http://www.ecs.umass.edu/ polizzi/feast ⇠

Contact: [email protected]

46 2012 Electronic Structure Workshop Wake Forest University

Graphene: Reversible Spin Manipulator of Molecular Magnet

S. Ghosh,1 S. Bhandary,2 H. Herper,3 H. Wende,3 O. Eriksson,2 and B. Sanyal2

1School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA. 2Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden. 3Faculty of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, 47048 Duisburg, Germany.

One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center by suitable external means. Fe-porphyrin (FeP) molecule is one of the potential candidate, where it is possible to tune the magnetic coupling between Fe at the center of FeP with a ferromagnetic substrate [1]. On the other hand, graphene has made an enormous sensation in the scientific community due to its exotic multidisciplinary features [2, 3]. During the synthesis of graphene a number of defects may arise, specially in sp2 bonded carbon the divacancy defect can easily be introduced in a graphene lattice [4]. These divacancy defect sites are excellent trapping centers for atom and molecules when dispersed on the graphene lattice [5]. Utilizing the novel properties of FeP and graphene, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S =1 S = 2, takes place for FeP molecule ! deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in di↵erent spin states, each with a unique saturation moment and easy axis orientation. The e↵ect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP [6].

[1] H. Wende et al., Nature Mater. 6, 516 (2007). [2] K. S. Novoselov et al., Science 306, 666 (2004). [3] A. K. Geim and K. S. Novoselov, Nature Mater. 6, 183 (2007). [4] M. H. Gass et al., Nature Nanotech. 3, 676 (2008). [5] B. Sanyal et al., Phys. Rev. B 79, 113409 (2009). [6] S. Bhandary et. al., Phys. Rev. Lett. 107, 257202 (2011).

Contact: [email protected]

47 2012 Electronic Structure Workshop Wake Forest University

Constructing an Improved Ce Pseudopotential in OPIUM Code

Z. K. Goldsmith, C.-W. Lee, and A. M. Rappe

The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA.

We construct a Ce pseudopotential in OPIUM that demonstrates excellent agreement with both experimentally and computationally determined physical properties of bulk ceria (Ce2O3 and CeO2) in density functional theory calculations. Our pseudopotential contains a mixture of semicore and partial core corrections, both of which are necessary to predict the correct energy level of 4f states. An a↵ectedly low energy for 4f results in unphysical interactions between Ce-4f and O-2p states in bulk Ce oxides. Our Ce pseudopotential re- produces the reaction enthalpy for the reduction of CeO2 to Ce2O3 with good agreement and similarly reproduces the activation barrier for bulk oxygen transport in pure CeO2. With the new pseudopotential, we investigate the energetics and kinetics of Pd-doped BaCeO3, a potential candidate of self-regenerative catalysts.

Contact: [email protected]

48 2012 Electronic Structure Workshop Wake Forest University

Interface Formation of Scandium Nitride on GaN(0001) Surface: A First-Principle Study

R. Gonz´alez-Hern´andez,1 W. L´opez-P´erez,1 J. A. Rodr´ıguez,2 and A. H. Romero3

1Grupo de Investigaci´onen F´ısica Aplicada, Departamento de F´ısica, Universidad del Norte, Barranquilla, Colombia. 2Grupo de Estudio de Materiales, Departamento de F´ısica, Universidad Nacional de Colombia, Bogot´a,Colombia. 3CINVESTAV, Departamento de Materiales, Unidad Queretaro, Queretaro 76230, Mexico.

Self-consistent periodic density functional theory calculations are used to investigate the role of scandium (Sc) impurity atoms during GaN growth. Adsorption and di↵usion of Sc on GaN(0001)-2 2 surface is examined and it is shown that Sc atoms preferentially adsorb ⇥ at the T4 sites at low and high coverages (from 1/4 up to 1 monolayer). It is also found that N atom is relatively immobile compared to Ga as well as Sc atoms on the GaN(0001) surface. In addition, calculating the relative surface energy of several configurations and various Sc concentrations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the Ga chemical potential. Based on these results, we found that incorporation of Sc adatoms in the Ga-substitutional site is energetically more favorable compared with the adsorption on the top layers. This e↵ect leads to the formation of an interfacial crystalline ScN layer on the GaN(0001) surface, which can o↵er a good interfacial combination between Sc and GaN substrate. Our calculations show that the scandium incorporation is most favorable under a nitrogen environment, in agreement with experimental results [1].

This work is funded by the Division de Investigaci´on, Desarrollo e Innovaci´on,Universidad del Norte, Barranquilla, Colombia.

[1] A. R. Kaplan, S. M. Prokes, S. C. Binari, and G. Kelner, Appl. Phys. Lett. 68, 23 (1996).

Contact: [email protected]

49 2012 Electronic Structure Workshop Wake Forest University

Metal-Insulator Transition of V2O3 from Screened Hybrid Functional

Y. Guo,1 S. J. Clark,2 and J. Robertson1

1Department of Engineering, Cambridge University, United Kingdom. 2Department of Physics, Durham University, United Kingdom.

The electronic structure of vanadium sesqui-oxide (V2O3) in di↵erent phases has been cal- culated by the screened exchange hybrid functional. After 40 years of experimental and theoretical investigation, V2O3 is still the subject of intense discussion because of the com- plicated phase transitions [1]. V2O3 is a canonical Mott-Hubbard insulator at low tem- perature, and can undergo several metal-insulator transitions by varying the temperature, pressure or doping. Here, di↵erent transitions have been examined by the screened exchange (sX) hybrid functional. This functional mixes a Thomas-Fermi screened Hartree-Fock ex- change into the local-density approximation (LDA) to correct the band gap and localization error of density functional theory [2]. As a generalized Kohn-Sham functional, sX can be used for energy minimization and structural relaxation. This allows a single shot calcula- tion of structure and electronic structure of systems in which bond length variations may contribute to the metal-insulator transition or to other complex phenomena such as multi- ferroic behavior. This contrasts with perturbative methods such as GW or dynamic mean field theory (DMFT) which only operate on a pre-defined structure. Here, we show that sX can reproduce the paramagnetic metal phase of V2O3 in the corundum structure, the anti- ferromagnetic insulating phase in the monoclinic structure, and the paramagnetic phase via Cr doping, without adjustable parameters. The electronic structure of the high tempera- ture metallic phase and the low temperature insulating phase is in good agreement with the photoemission spectra [3]. Furthermore we have relaxed a Cr-doped V2O3 corundum struc- ture supercell to realize the doping induced paramagnetic insulating phase with a 0.15eV energy gap from first principle calculation for the first time. We show that the metal insu- lator transition induced by crystal structure change and doping could be explained in the framework of band theory. The hybrid sX functional can describe the electronic structure change when the Hubbard correlation energy is comparable to the band energy.

This work is funded by the European Commisson.

[1] S. Lupi et al., Nat. Commun. 1, 105 (2010); S. Y. Ezhov, V. I. Anisimov, D. I. Khomskii, and G. A. Sawatzky, Phys. Rev. Lett. 83, 4136 (1999). [2] S. J. Clark and J. Robertson, Phys. Rev. B 82 085208 (2010). [3] S. K. Mo et al., Phys. Rev. B 74, 165101 (2006); H. Fujiwara et al., Phys. Rev. B 84, 075117 (2011).

Contact: [email protected]

50 2012 Electronic Structure Workshop Wake Forest University

First Principles Computer Simulations of Li10GeP2S12 and Related Lithium Superionic Conductors

N. A. W. Holzwarth

Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA.

A recent paper by Kamaya et al. [1] reported a new crystalline superionic conductor having a compact tetrahedral structure and a stoichiometry of Li10GeP2S12. The room temperature conductivity was reported to be 0.01 S/cm, comparable to liquid electrolyte conductivies and five times higher the compositionally related thio-LISICON material Li3.25Ge0.25P0.75S4 developed earlier [2]. This poster presents a progress report on our work to perform first principles computer simulations for these materials, focussing on the structural, stability, and Li ion mobility properties of idealized crystalline models. From the perspective of our previous studies of Li ion conductivity in lithium thiophosphate electrolytes [3, 4], the e↵ects of introducing Ge can be assessed.

Supported by NSF Grants DMR-0705239 and DMR-1105485.

[1] N. Kamaya et al., Nature Materials 10, 682 (2011). [2] R. Kanno et al., J. Electrochem. Soc. 148, A742 (2001). [3] N. A. W. Holzwarth et al., J. Power Sources 196, 6870 (2011). [4] N. D. Lepley et al., J. Electrochem. Soc. 159, A538 (2012).

Contact: [email protected]

51 2012 Electronic Structure Workshop Wake Forest University

Spin-Phonon Coupling E↵ects in Transition-Metal Perovskites: A DFT+U and Hybrid-Functional Study

J. Hong,1 A. Stroppa,2 J. ´I˜niguez,3 S. Picozzi,2 and D. Vanderbilt1

1Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854-8019, USA. 2CNR-SPIN, L’Aquila, Italy. 3Institut de Ci`encia de Materials de Barcelona, Campus UAB, 08193 Bellaterra, Spain.

Spin-phonon coupling e↵ects, as reflected in phonon frequency shifts between ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic CaMnO3, SrMnO3, BaMnO3, LaCrO3, LaFeO3 and La2(CrFe)O6, are investigated using density-functional methods [1]. We find that the phonon frequency shifts ! = ! ! strongly de- AFM FM pend on the U value when using the DFT+U method. We propose a scheme to obtain a proper value for U by fitting to hybrid-functional (HSE) calculations of energy di↵erences between states of di↵erent magnetic order. The phonon frequency shifts obtained in this way agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational e↵ort. We find that in the AMnO3 materials class with (A=Ca,Sr,Ba), the (R) phonon frequency shift ! decreases (increases) as the 2+ A size increases. In LaMO3 (M=Cr, Fe, Cr/Fe), the phonon frequencies at decrease as spin order changes from AFM to FM for LaCrO3 and LaFeO3, but they increase for double perovskite La2(CrFe)O6. We discuss the prospects for bulk and superlattice forms of these materials to be useful as multiferroics.

This work was supported by ONR Grant 00014-05-0054, by Grant Agreement No. 203523-BISMUTH of the EU-FP7 European Research Council, and by MICINN-Spain (Grants No. MAT2010-18113, No. MAT2010-10093-E, and No. CSD2007-00041). Computations were carried out at the Center for Piezoelectrics by Design.

[1] J. Hong, A. Stroppa, J. Iniguez, S. Picozzi, and D. Vanderbilt, Phys. Rev. B 85, 054417 (2012).

Contact: [email protected]

52 2012 Electronic Structure Workshop Wake Forest University

Rationale for the High Reactivity of the Interfacial

Sites in Methanol Oxidation on Au/TiO2⇤

S. Hong and T. S. Rahman

Department of Physics, University of Central Florida, Orlando, FL 32816, USA.

The role of perimeter/interfacial sites of titania supported gold nanoparticles have recently been the subject of discussion in several experimental and theoretical studies because of their high activity in various reactions. We have carried out density functional theory based calculations to find that the high activity of these active sites in Au/TiO2 towards methanol reaction originates from the charge transfer induced Coulomb interaction among gold, reactant, and reducible TiO2 through the formation of ionic O-Au bond between gold atoms and methoxy in the active sites. A direct result of such charge transfer induced repulsion is tilting of the methoxy axis, which leads to facile reaction of methoxy with bridge oxygen atoms readily available from the reducible support through C-H scission. We will present details of the charge density redistributions, projected electronic density of states and calculated activation energy barriers and reactions paths which may control the selectivity of the reaction towards the formation of formaldehyde and compare them with experimental data from the Chen group [1].

Work supported in part by DOE grant DE-FG02-07ER15842.

[1] D. A. Chen et al., to be published.

Contact: [email protected]

53 2012 Electronic Structure Workshop Wake Forest University

CsSnX3 Band Structure Calculations by the QSGW Method

L.-Y. Huang and W. R. L. Lambrecht

Department of Physics, Case Western Reserve University, Cleveland, OH 44106, USA.

CsSnX3 perovskite compounds are of interest because of their strong photoluminescence. Their electronic properties are not yet well understood. Here, we carry out quasiparticle self-consistent GW (QSGW) calculation for cubic (↵-phase) CsSnX3, where X can be a Cl, Br or I ion. Our results show larger band gaps then previous calculations using only local density approximation (LDA) or generalized gradient approximation (GGA) [1, 2]. Our QSGW gaps are in good agreement with experiment. An analysis of the orbital character of the bands, shows that the valence band maximum has a strong Sn-s X-p antibonding character, while the conduction band minimum has Sn-p character. The direct gap thus has a strong intra-atomic Sn-s to Sn-p character which explains to first order why these materials have strong luminescence. Our calculations for Cl, Br and I show that the variation of the anion only has a moderate e↵ect on the band gaps. We also performed GW calculations for the tetragonal (-phase) CsSnI3 in order to study the band gap changes during the structural phase transition. It turns out that the QSGW correction from LDA are only weakly dependent on the structure. We thus can estimate the band gap of the orthorhombic (-phase) CsSnI3 using the LDA results. The spin-orbit coupling e↵ect is included in our calculations.

This work is funded by NSF.

[1] I. Borriello, G. Cantele, and D. Ninno, Phys. Rev. B 77, 235214 (2008). [2] J.-F. Chabot, M. Cote, and J.-F. Briere. Ab-initio study of the electronic and struc- tural properties of CsSnI3 perovskite. In D. Senechal, editor, High Performance Com- puting Systems and Applications and OSCAR Symposium (Proceedings). NRC Re- search Press, Ottawa, 2003.

Contact: [email protected]

54 2012 Electronic Structure Workshop Wake Forest University

Lattice Vibrational Modes in Si/Ge Core-shell Nanowires

S. Huang and L. Yang

Department of Physics, Washington University, Saint Louis, MO 63130, USA.

We present a first-principles study on lattice vibrational modes of Si/Ge core-shell nanowires (NWs). We particularly focus on two types of modes of broad interest, the high-frequency optical modes and the radial breathing mode (RBM). Those optical modes exhibit sub- stantial energy shifts as the structure of core-shell NWs varies, which can be attributed to the change of the strain condition. Moreover, those studied optical modes and their energy shifts are significant in the calculated Raman scattering spectrum, providing a convenient way to detect the structural information of core-shell nanomaterials. Meanwhile, the fre- quency of the RBM also strongly depends on the structure of core-shell NWs, which can be explained by an elastic media model. In particular, we find that the di↵erence between sound velocities of core and shell are enhanced by the internal strain, which could be useful for thermoelectric applications based on core-shell nanostructures.

Contact: [email protected]

55 2012 Electronic Structure Workshop Wake Forest University

Nanoscale Photovoltaics: Aminoethanethiol Coated CdSe Quantum Dots

J. Jiang and S. Ismail-Beigi

Department of Applied Physics, Yale University, New Haven, CT 06520, USA.

Ordered heterojunction solar cells show promise for the development of next generation high eciency photovoltaics due to their optimized carrier transport properties. One approach is to use colloidal CdSe quantum dots (QD) to decorate long and aligned carbon nanotubes. In principle, this choice combines the tunable band gap and multiple exciton generation of the QDs with the high carrier mobility of carbon nanotubes. One diculty involves the proper surface capping of the QD and its attachment to the nanotube. QD capping agents typical consist of long carbon chains which form a barrier to charge carriers and hinder charger transfer. Short ligands such as aminoethanethiols and pyridines are thus a present focus of activity. Using first principles, we provide insights into binding of the aminoethanethiol molecules to the QD surface. The linking end of the aminoethanethiols contains a sulfur atom with an unpaired electron: the sulfur acts either as a hole donor to the QD when a Cd-S bond forms or as an electron donor when a S-Se bond forms. While the S-Cd bond is stronger in isolation, the preferred binding morphology for two ligands involves both S-Cd and S-Se bonds. We discuss these and related issues and their implications.

This work is funded by the NSF.

Contact: [email protected]

56 2012 Electronic Structure Workshop Wake Forest University

Magnetic Properties of Transition Metal Nanoparticles: A DFT-Inhomogeneous-DMFT Analysis

A. Kabir, V. Turkowski, and T. S. Rahman

Department of Physics, University of Central Florida, Orlando, FL 32816, USA.

To include electron-electron correlation e↵ects in the nanosystem we are proposing a com- bined density-functional-theorydynamical-mean-field-theory (DFT + DMFT) approach, which we have recently shown to be suitable for including correlation e↵ects in small (2-5 atoms) Fe and FePt clusters [1]. This method has several advantages as compared to the widely used DFT + U approach, the most important of which is that it takes into account dynamical correlation e↵ects automatically. We find that the inclusion of dynamical ef- fects leads to a reduction in the cluster magnetization (as compared to results from DFT + U ) and that, the magnetization values agree well with experimental estimations. We will present results of application of this formalism to examine the size dependent magnetic properties of Fe15,Fe17 and Fe19 clusters. For the above we have developed a computational code that should be capable of calculating the magnetic properties of systems containing hundreds of atom.

Work supported in part by DOE Grant No. DOE-DE-FG02-07ER46354.

[1] V. Turkowski, A. Kabir, N. Nayyar, and T. S. Rahman, J. Phys.: Cond. Matter 22, 462202 (2010); J. Chem. Phys. 136, 114108 (2012).

Contact: [email protected]

57 2012 Electronic Structure Workshop Wake Forest University

Ferroelectric Surface Chemistry: First-Principle Study of NOx Decomposition

A. Kakekhani1 and S. Ismail-Beigi1,2

1Department of Physics, Yale University, New Haven, CT 06520, USA. 2Department of Applied Physics, Yale University, New Haven, CT 06520, USA.

NOx molecules are critical and regulated air pollutants produced during automotive com- bustion. As part of long-term e↵ort to design viable catalysts for NOx decomposition that operate at higher temperatures, and thus would allow for greater fuel eciency, we are studying the basic science of NOx chemistry on ferroelectric perovskite surfaces. Chang- ing the direction of the ferroelectric polarization can modify surface electronic properties and thus can lead, in principle, to switchable surface chemistry. Here, we begin with the question of how NO and NO2 molecules behave on the polar (001) surfaces of ferroelectric PbTiO3 as function of ferroelectric polarization, surface stoichiometry, and various molecu- lar or dissociated binding modes. We will discuss our calculated polarization and coverage dependences based on the electronic structure of the surface system.

This work is funded by the Toyota Motor Engineering and Manufacturing, North America, Inc.

Contact: [email protected]

58 2012 Electronic Structure Workshop Wake Forest University

Adsorption Characteristics of Acenes on Cu(110) and Ag(110)

A. Kara

Department of Physics, University of Central Florida, Orlando, FL 32816 USA.

Using density functional theory and the generalised gradient approximation, we have cal- culated the adsorption characteristics of five acenes (Benzene, Naphtalene, Anthracene, Tertracene and Pentacene) on the (110) surface of copper and silver. We found the binding energy to increase with number of carbon atoms in the molecule for both silver and copper cases. However, the increase in the binding energy is substantially large for the case of copper as compared to the case of silver. In the case of adsorption on Ag(110), all geomet- rical and electronic structural changes point to physisorption for all acenes, while for the adsorption on Cu(110), these characteritics show a gradual shift from physisorption (from Benzene to Naphtalene) to chemisorption for Tetracene and Pentacene with the border case of Anthracene. The characteristics to distinguish between physisorption and chemisorption used in the present study are: bending and tilting of the molecule, buckling in the substrate top-most layers, charge transfer, change in the workfunction, d-band center and width, and finally the existence of interface states near the Fermi level.

This work is funded by the Department of Energy with Grant No. DE-FG02-11ER16243.

Contact: [email protected]

59 2012 Electronic Structure Workshop Wake Forest University

Core Polarization E↵ects from Dielectric Eigenpotentials

A. Kaur,1 E. R. Ylvisaker,1 D. Lu,2 T. A. Pham,3 G. Galli,1,3 and W. E. Pickett1

1Department of Physics, University of California, Davis, CA 95616, USA. 2Center for Functional Nanomaterials, BNL, Upton, NY 11973, USA. 3Department of Chemistry, University of California, Davis, CA 95616, USA.

The dielectric function eigenpotential representation is used to investigate various conse- quences when core polarization is included with the valence electron polarization. The semicore leads to additional dielectric screening which becomes evident in the extra bands in the dielectric band structure in solids and in additional eigenpotential in molecules and atoms. We illustrate how (semi)core contributions influence dielectric screening for phonons and the dielectric constant in ionic crystals, and for quasiparticle energies at the G0W0 level. We conclude that in a variety of applications it is crucial to include (semi)core electrons to describe adequately dielectric screening and its consequences, for both static and dynamical behavior.

Contact: [email protected]

60 2012 Electronic Structure Workshop Wake Forest University

Real-Space All-electron Band Structure Calculations

J. Kestyn and E. Polizzi

Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA 01003, USA.

We are presenting an all-electron numerical framework for band structure calculations. A 3D finite element mesh is used to discretize the DFT/Kohn-Sham problem in real space using full ionic local potential (without introducing pseudopotentials). The mathematical model allows flexibility for addressing various combinations of Bloch and Dirichlet bound- ary conditions for low-dimensional nanostructures, and the resulting large-scale eigenvalue problems are solved using the FEAST solver [1]. The FEAST algorithm [2] o↵ers many important and unique capabilities for achieving accuracy, robustness, high-performance and scalability on parallel computing architectures. At first, the algorithm can operate in par- allel to obtain core and valence electrons independently spanning di↵erent energy ranges. Secondly, solving the original eigenvalue problem within a given energy range (i.e. search interval) is mainly reformulated into solving a set of well-defined independent linear systems along a complex energy contour. Additionally, a mun-tin domain decomposition can be used directly to solve the resulting linear systems [3] without the need to reformulate the original eigenvalue problem into a non-linear one (as it the case using the APW technique [4]). The complexity of numerical framework scales linearly with the number of atoms, and it is flexible to address arbitrary impurities, defects and roughness. Numerical simulation results using DFT/LDA are first presented for polyparaphenylene (PPP) [5], which are in strong agreement with [6]. Band structure calculations of other graphene nanoribbon con- figurations (n-AGNR) and carbon nanotubes (n-n CNT) will then also be presented and discussed.

This work is funded by the NSF 0846457.

[1] http://www.ecs.umass.edu/ polizzi/feast ⇠ [2] E. Polizzi, Phys. Rev. B 79, 115112 (2009). [3] A. Levin, D. Zhang, E. Polizzi, submitted (2012), http://arxiv.org/abs/1106.3609. [4] J. C. Slater, Phys. Rev. 51, 846 (1937). [5] M.S. Miao, P. E. Van Camp, V. E. Van Doren, J. J. Ladik, and J. W. Mintmire, J. Chem. Phys. Vol. 109, 9623 (1998).

Contact: [email protected]

61 2012 Electronic Structure Workshop Wake Forest University

Strong Dependence of Oxide Work Functions on Surface Structures

S. Kim, C. Lee, and A. M. Rappe

The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA.

Using density functional theory calculations, we find that work functions of oxides, even for the same surface orientations, strongly depend on both composition and geometry of the surfaces. Oxide surfaces often change their compositions and/or geometric structures depending on the environmental conditions in which they are exposed or prepared, and those deviations might cause significant di↵erence in electronic properties. From atomic- level investigations on various reconstructed surfaces of SrTiO3 (001), BaTiO3 (001), TiO2 (110) and LaFeO3 (010), we have observed that work functions can be changed up to 5 eV when a surface undergoes compositionally di↵erent reconstruction. Moreover, we have observed that, with some stoichiometrically identical surfaces, geometric change can alter the work functions significantly. This work function change can be interpreted partially with surface states, and more importantly, with the potential depth of bulk region modified by surface dipole layer. Our finding reflects the importance of the surface structure on the electron transfer phenomena at oxide surfaces.

Contact: [email protected]

62 2012 Electronic Structure Workshop Wake Forest University

Teaching Computers to be Physicists: Machine Learning with Network Functions

B. Kolb and T. Thonhauser

Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA.

It is often the case that a physicist must find a mathematical relationship between a set of independent variables and some observable. This can be carried out by fitting the data to some assumed form, but machine learning techniques can be used to carry out the same task with more generality. Network functions are computational networks designed to allow the techniques of machine learning to be used to find mathematical relationships buried in a set of data. Network functions use composition of fundamental mathematical operations, to represent complex analytical functions. This is done through a continuously variable functional form, governed by a set of trainable parameters. The precise form taken on by a network can be learned using simple training algorithms. Unlike neural networks, many target functions can be learned exactly over their entire domain by a finite—and often small—network. In the worst case, a network can learn to approximate a target function to arbitrary precision within some range of inputs. There are many uses for network functions, including searching for a kinetic energy density functional or an improved exchange-correlation functional within density functional theory.

Contact: [email protected]

63 2012 Electronic Structure Workshop Wake Forest University

Ab Initio Spectroscopy and Screening of Materials for the Li/Air Batteries

B. Kozinsky and R. S. Sanchez-Carrera

Research and Technology Center North America, Robert Bosch LLC, Cambridge, MA 02142, USA.

The Li/air battery has recently received much attention because of its high theoretical energy density. However, there are still many technical problems impeding the development of a practical Li/air battery; even a basic understanding of the molecular reactions governing this electrochemical system remains elusive. We have used first-principles calculations to improve our understanding of the molecular reaction mechanism and to investigate the Raman and NMR spectroscopic signatures of possible Li/air reaction products/by-products. Our simulations of candidate molecules indicate that specific spectroscopic characteristics can be used to determine the structure and composition of the discharge products from measured spectra. The electrochemical stability of the solvent is another challenge, a↵ecting reversibility of the Li/air battery. We identify a set of descriptors of electrochemical stability based on total energy computations. Subsequently we perform systematic screening of a large database of potential candidate solvent molecules, and extract useful design rules and trade-o↵relationships.

Contact: [email protected]

64 2012 Electronic Structure Workshop Wake Forest University

Two-Level States in Substitutional Mixed Crystal: Ab Initio Predictions for Glass Transitions

J. Lee and T. A. Arias

Laboratory of Atomic and Solid State Physics, Cornell University Ithaca, NY, USA.

The nature of glass transitions has been a subject of great interest in solid state community due to their universal characteristics ccurring for a large range of materials. Among many explanations for the origin of this universality, the two-level states (TLS) theory [1, 2] stands as the simplest but most successful one. However, the identities of the two-level states have been confirmed for only few systems, and even so, only at the level of molecular dynamic simulation, due to a large computational cost of ab initio calculations. Here we presents an ab initio investigation for the nature of the two-level states in one of the substitutional mixed crystals observed experimentally to be a glassy material [3] , Ba1 x CaxF2. We determine the underlying mechanism of the formation of the two-level states, which is the symmetry breaking in crystal structure driven by chemically induced local strains. A modulated Landau theory reproduces all the ab initio simulation results, and leads us to a prediction for the spectral density of states in very good agreement with the experiment.

[1] P. W. Anderson, B. I. Halperin, and C. M. Varma, Phil. Mag. 25, 1 (1972). [2] W. A. Phillips, Rep. Prog. Phys. 50, 1657 (1987). [3] J. P. Wrubel et al., Phys. Rev. Lett. 96, 235503 (2006).

Contact: [email protected]

65 2012 Electronic Structure Workshop Wake Forest University

Computer Modeling of Crystalline Electrolytes- Lithium Thiophosphates and Phosphates

N. D. Lepley and N. A. W. Holzwarth

Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA.

Recently, lithium thiophosphate materials suitable for usage as solid electrolytes in Li- ion battery applications have been developed. These materials possess room temperature 3 ionic conductivities as high as 10 S/cm [1], 3 orders of magnitude greater than that of commercial solid electrolyte materials based on LiPON [2]. The most promising of these thiophosphates, a superionic conductor with stoichiometry Li7P3S11 [1], is investigated in this work using first principles calculations. In addition to examining the stability and structure of this material, we analyze in detail the migration mechanisms for both Li7P3S11 and for hypothetical LiPON-like phosphate and phosphonitride analogues. Our results [3] correlate well with experimental findings and o↵er an explanation for the high conductivity observed in Li7P3S11.

This work was funded by NSF grant DMR-0705239.

[1] F. Mizuno, A. Hayashi, K.Tadanga, and M. Tatsumiago, Solid State Ionics 177, 2721 (2006). [2] N. J. Dudney, Interfaces 17, 44 (2008). [3] N. D. Lepley and N. A. W. Holzwarth, J. Electrochem. Soc. 159, A538 (2012).

Contact: [email protected]

66 2012 Electronic Structure Workshop Wake Forest University

Surface Studies with Combined Free Energy Functionals of Electronic and Liquid Densities

K. Letchworth-Weaver, R. Sundararaman, and T. A. Arias Department of Physics, Cornell University, Ithaca, NY 14853, USA.

The microscopic structure of both a solid surface and a contacting liquid can be dramatically a↵ected by the interaction between the two systems, particularly at the interface between a polar surface and a polar liquid. We present a study of oxide and metallic surfaces in a liquid environment with Joint Density Functional Theory (JDFT) [1], a computationally ecient alternative to molecular dynamics simulations which replaces thermal sampling with a single variational principle for the free energy of the full system. Within the rigorous framework of JDFT, we introduce classical density-functionals for ionic species and describe how to couple them with existing functionals for liquid water and traditional electronic density-functionals. Calculations were performed within the open source software package JDFTx [2] and employ a liquid water functional, which captures bulk properties and microscopic structure over the entire phase diagram of the liquid [3], and a density-only coupling functional between the electronic and liquid systems [4]. With this microscopically accurate description of the liquid-solid interface structure, we study the voltage-dependent behavior of transition metal electrodes in an aqueous electrolyte environment and show qualitative agreement with experiment [5]. We additionally predict the structure of water at a rutile TiO2 (001) surface and investigate the e↵ects of solvation on the electronic properties of the system. In each case, we gain physical insight which could direct future studies of catalysis and electrode stability in electrochemical systems.

This material is based on work supported by the Energy Materials Center at Cornell, an Energy Frontier Research Center funded by the U.S. Department of Energy, Oce of Science, Oce of Basic Energy Science under Award Number DE-SC0001086. K. Letchworth-Weaver additionally acknowledges support from an NSF Graduate Research Fellowship.

[1] S. A. Petrosyan, A. A. Rigos, and T. A. Arias, J. Phys. Chem. B 109, 15436 (2005). [2] R. Sundararaman, K. Letchworth-Weaver, and T. A. Arias, “JDFTx,” http://jdftx.sourceforge.net (2012). [3] J. Lischner and T. A. Arias, J. Phys. Chem. B 114, 1946 (2010). [4] K. Letchworth Weaver, R. Sundararaman, and T. A. Arias, “A Priori Method for First Principles Study of Aqueous Electrochemistry,” Presented at the APS March Meeting (2011). [5] K. Letchworth Weaver, R. Sundararaman, and T. A. Arias, “Surface Studies with Combined Free Energy Functionals of Electronic and Liquid Densities,” Presented at the APS March Meeting (2012).

Contact: [email protected]

67 2012 Electronic Structure Workshop Wake Forest University

Enhanced Many-Electron E↵ects in Gated Bilayer Graphene

Y. Liang and L. Yang

Department of Physics, Washington University, St. Louis, MO 63136, USA.

Our first-principles GW-Bethe-Salpeter Equation (BSE) simulation reveals that many- electrons e↵ects are crucial to decide the electronic structure and optical excitations of gated bilayer graphene (GBLG), a two-dimensional narrow-gap semiconductor. Enhanced electron-electron interactions dramatically enlarge the quasiparticle band gap; the infrared optical absorption spectra are dictated by a bright bound exciton, which well explains re- cent experiments. Moreover, our calculation predicts an unusual low-energy dark exciton whose electron and hole are condensed onto two graphene layers, respectively, providing a neat object to study entangling e↵ects of electron-hole pairs. Since all aforementioned many-electron e↵ects can be eciently tuned by the applied gate field, our calculation paves the foundation for optoelectronic applications based on GBLG.

[1] E. V. Castro et al., Phys. Rev. Lett. 99, 216802 (2007).

Contact: [email protected]

68 2012 Electronic Structure Workshop Wake Forest University

Transport of Hot Electrons in Scintillators: NaI and SrI2

Q. Li,1 J. Q. Grim,1 R. Williams,1 D. Aberg,2 and B. Sadigh2

1Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA. 2Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.

The hot electrons induced by incident gamma-ray in scintillators lose their energy from one energy gap (EG) above to conduction band minimum (CBM) though various scattering channels, among which the most important one is phonon scattering [1]. Hence, the trans- port of hot electrons should be characterized by mainly their group velocities and optical phonon energies. We present results for a density functional theory study of the electron group velocities of two halide scintillators: NaI and SrI2. The holes in both materials are ac- cepted to self-trap less than several picoseconds, forming a core of self-trapped holes (STH) with a radius of about 3nm. It is demonstrated that the inclusion of full band structure significantly improves the result with respect to the prediction of a free electron gas model. A recent model of quenching and transport showed that linear quenching by trapping on defects a↵ects the both light yield and nonproportionality of scintillators [2]. Our calcula- tion shows that the average group velocity of NaI is about 5 times greater than that of SrI2. Taking their similar optical phonon energies into consideration, we expect that the hot elec- trons in NaI cover a much bigger range in space through transport before thermalization. Therefore, we expect greater chance of getting trapped on deep defects for the thermalized electrons of NaI during their hopping walk back toward the STH core for recombination, which causes less relative light yield and more nonproportionality. Furthermore, we deduce that this tread can be generalized to explain the performance of two di↵erent scintillator material families: monovalent halides which in general have simple unit cells and bigger group velocities and multivalent halides which in general have relatively complicated unit cells and smaller group velocities. These results suggest a design parameter for scintillator materials.

Supported by the Oce of Nonproliferation Research and Development (NA-22) of the U.S. Department of Energy under contracts DE-NA0001012 (Fisk-WFU), DE-AC02-05CH11231 (LBNL-WFU), and DE-AC52-07NA27344 (LLNL).

[1] Z. Wang et al., J. Appl. Phys., in press (2012). [2] Q. Li, J. Q. Grim, R. T. Williams, G. A. Bizarri, and W. W. Moses, J. Appl. Phys. 109, 123716 (2011).

Contact: [email protected]

69 2012 Electronic Structure Workshop Wake Forest University

First Principles Calculations of Iodine Vacancy Centers in SrI2

Q. Li,1 R. Williams,1 D. Aberg,2 and B. Sadigh2

1Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA. 2Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.

2+ SrI2:Eu is a scintillator that has recently attracted intense interest because of its excep- tional light yield and proportionality properties. However, partly because it is hygroscopic and dicult to study by routine experimental techniques, there is relatively little known of some of its properties, including lattice defects. A recent model of quenching and trans- port showed that linear quenching by trapping on defects as well as nonlinear quenching of dense electron-hole distributions along the primary electron track can interact with carrier transport in the track gradient to a↵ect nonproportionality [1]. Thus information on the important lattice defects in SrI2 is of interest for development of this material as a high- performance scintillator. We present the electronic structure and the transition energies of iodine vacancy defects in SrI2 for several charge states. We use both semilocal (GGA-PBE) and hybrid functionals, as it is commonly accepted that hybrid functionals can improve accuracy of the band gap and hence relevant energy levels. The finite-size supercell e↵ect is investigated in the Makov-Payne scheme [2]. Comparison, where appropriate, is made to published results on chlorine vacancy defects in NaCl calculated with similar methods and functionals [3]. Experimental searches for lattice defects and their properties in the SrI2 family of materials are underway in Wake Forest, Fisk, Berkeley, and Livermore labs among others, and comparison to the calculations will be discussed to the extent that experimental results have become available.

Supported by the Oce of Nonproliferation Research and Development (NA-22) of the U.S. Department of Energy under contracts DE-NA0001012 (Fisk-WFU), DE-AC02-05CH11231 (LBNL-WFU), and DE-AC52-07NA27344 (LLNL).

[1] Q. Li, J. Q. Grim, R. T. Williams, G. A. Bizarri, and W. W. Moses, J. Appl. Phys. 109, 123716 (2011). [2] Q. Makov and M. C. Payne, Phys. Rev. B 51, 4014 (1995). [3] W. Chen, C. Tegenkamp, and H. Pfnur, Phys. Rev. B 82, 104106 (2010).

Contact: [email protected]

70 2012 Electronic Structure Workshop Wake Forest University

Edges of Low-dimensional Materials: Structures, Energies, and Applications

Y. Liu, V. I. Artyukhov, X. Zou, and B. I. Yakobson

Department of Mechanical Engineering and Materials Science, Department of Chemistry, and the Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, TX 77005, USA.

Edges and interfaces play important roles in the morphologies and properties of low- dimensional materials. Based on first-principles calculations, we present that (i) the energies of graphene edge, and its implication to nanotube chirality [1]; (ii) the thermodynamics and kinetics of graphene growth is determined by its edge [2]; (iii) the interfacial structures and properties of merging graphene domains, i.e. grain boundaries [3]; (iv) the structures and properties of grain boundaries in two-dimensional (2D) boron nitride (BN) [4]; (v) the en- ergies of single crystalline 2D BN edges, and interfaces of graphene quantum dot embedded in BN matrix with electronic and magnetic properties modulation [5]; (vi) The crack propa- gation is determined by energies of graphene edge [6]. Our studies enrich the understanding of edges and interfaces in low-dimensional materials.

[1] Y. Liu, A. Dobrinsky, and B. I. Yakobson, Phys. Rev. Lett. 105, 235502 (2010). [2] V. I. Artyukhov, Y. Liu, and B. I. Yakobson, submitted (2012). [3] Y. Liu and B. I. Yakobson, Nano Lett. 10, 2178 (2010). [4] Y. Liu, X. Zou, and B. I. Yakobson, in preparation (2012). [5] Y. Liu, S. Bhowmick, and B. I. Yakobson, Nano Lett. 11, 3113 (2011). [6] K. Kim, V. I. Artyukhov, W. Regan, Y. Liu, M. F. Crommie, B. I. Yakobson, and A. Zettl, Nano Lett. 12, 293 (2012).

Contact: [email protected] & [email protected]

71 2012 Electronic Structure Workshop Wake Forest University

Ab Initio Calculation of Indirect Spin-Spin Coupling Constants

M. G. Lopez and T. Thonhauser

Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA.

Indirect spin-spin, or “J-coupling” constants have been of considerable use for extracting de- tailed structural information from high-resolution NMR spectra. We present here a method for calculating all four contributions from the nonrelativistic Ramsey theory; the dia- and para-magnetic contributions along with the Fermi-contact and spin-dipolar contributions. Our method is based on the so-called “converse” method for NMR [1] and computes the nec- essary energy second derivatives directly using a numerical finite-di↵erences scheme, there- fore eliminating the need for a complex linear response framework. We have implemented our method as part of a plane-wave density functional theory code using pseudopotentials.

[1] T. Thonhauser et al., Phys. Rev. B 81, 184424 (2010).

Contact: [email protected]

72 2012 Electronic Structure Workshop Wake Forest University

Ab-Initio Study of the Structural and Electronic Properties of Zr Adsorption on AlN (0001) Surface

W. L´opez-P´erez,1 R. Gonz´olez-Hern´andez,1 J. Rivera-Julio,1 G. E. Escorcia-Salas,2 and J. Sierra-Ortega2

1Grupo de F´ısica Aplicada, Departamento de F´ısica, Universidad del Norte, Barranquilla, Colombia. 2Grupo de Investigaci´onen Teor´ıade la Materia Condensada, Universidad del Magdalena, Santa Marta, Colombia.

We have performed spin-polarized first-principles calculations to explore the Zr adsorption and di↵usion on the AlN(0001)-(2 2) surface. The calculations were performed using the ⇥ generalized gradient approximation (GGA) [1] with ultrasoft pseudopotential [2] within the density functional theory (DFT) [3]. We found that the most energetically favorable structure corresponds to the Zr-T4 reconstruction or the Zr adatom located at the hcp- hollow (T4) site, while the Zr adsorption on-top of a aluminium atom (T1 position) is energetically unfavorable. The Zr di↵usion on surface shows an activation energy of 0.473 eV (T to H ). The resultant reconstruction of the Zr adsorption on AlN(0001)-2 2 surface 4 3 ⇥ presents a lateral relaxation of some hundredth of Ain˚ the most stable site. The comparison between the electronic structure of the AlN(0001) clean-surface and with Zr adatom is also examined. The clean surface aluminium-terminated has one Al dangling-bond in the top layer. We found that the clean AlN(0001) surface layer have metallic behavior, due to the partial occupation of the Al dangling-bond band by 0.75 of an electron, disappearing the existing gap in the bulk AlN. The existence of a partially filled state means that this state gives rise to a metallic surface with the Fermi energy crossing the surface state. The Zr adsorption saturates some dangling bonds of the clean surface and the system presents a metallic behavior in both spin components.

This work is funded by the Divisi´onde Investigaci´on,Desarrollo e Innovaci´on, Universidad del Norte, Barranquilla, Colombia.

[1] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. B 77, 3865 (1996). [2] D. Vanderbilt, Phys. Rev. B 41, 7892 (1990). [3] P. Hohenberg and W. Kohn, Phys. Rev. 136, 864 (1964).

Contact: [email protected]

73 2012 Electronic Structure Workshop Wake Forest University

Beyond RPA Correlation Energies: Evaluation of Model Exchange-Correlation Kernels

D. Lu

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA.

The adiabatic-connection fluctuation-dissipation theorem (ACFDT) provides a formal frame work to treat the exchange-correlation energy. Under the random phase approximation (RPA), the e↵ect of the exchange-correlation kernel is neglected, leading to the so-called EXX/RPA method. It has been shown that EXX/RPA yields overall a good description of structural properties of materials with di↵erent dimensionalities and di↵erent types of chem- ical bonds. In particular, the description of van der Waals systems is significantly improved over the density functional theory in the local or semi-local approximations. However, due to the lack of exchange-correlation kernel, EXX/RPA overestimates absolute correlation energy and slightly underestimates the cohesive energy. In this work, we applied the full ACFDT by including model exchange-correlation kernels. Two types of model kernels were studied: a local kernel proposed by Dobson and Wang [1] and a non-local kernel proposed by Corradini et al. [2]. We generalized the original Corradini kernel to inhomogeneous systems, and developed a new algorithm to allow an ecient numerical implementation. Results on benchmark systems were discussed.

Work at Brookhaven supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy.

[1] J. F. Dobson and J. Wang, Phys. Rev. B 62, 10038 (2000). [2] M. Corradini, R. Del Sole, G. Onida, and M. Palummo, Phys. Rev. Lett. 57, 14569 (1998).

Contact: [email protected]

74 2012 Electronic Structure Workshop Wake Forest University

Spin-orbit Coupling within Dynamical Mean-Field Theory: Coulomb Correlations in 4d and 5d Transition Metal Oxides

1,2, 3 1,2 C. Martins, ⇤ M. Aichhorn, and S. Biermann

1Centre de Physique Th´eorique, Ecole Polytechnique, CNRS, 91128 Palaiseau Cedex, France. 2Japan Science and Technology Agency, CREST, Kawaguchi 332-0012, Japan. 3Institute of Theoretical and Computational Physics, TU Graz, Graz, Austria. ⇤New address: CEA, DAM, DIF, F-91297, Arpajon, France.

We present an approach that extends the combined local-density approximation (LDA) and dynamical mean-field theory (DMFT) scheme to the case of materials with large spin- orbit (SO) interactions. The LDA+DMFT implementation developed in the framework of the full-potential linear augmented plane-wave method by Aichhorn et al. [1] can now be applied to systems where the correlated Wannier orbitals must be built out of local orbitals defined in the strong spin-orbit coupling limit. The screened Coulomb interaction and Hund’s coupling are evaluated consistently from a first-principles constrained random- phase approximation scheme [2]. Using this LDA+SO+DMFT implementation, in conjunction with a continuous-time quantum Monte Carlo algorithm, we investigate the electronic excitations in the param- agnetic phases of strontium iridate (Sr2IrO4) and strontium rhodate (Sr2RhO4). We show that the interplay of spin-orbit interactions, structural distortions and Coulomb interactions suppresses spin-orbital fluctuations. As a result, the room temperature phase of Sr2IrO4 is a paramagnetic spin-orbitally ordered Mott insulator. In Sr2RhO4, the e↵ective spin-orbital degeneracy is reduced, but the material remains metallic, due to both, smaller spin-orbit and smaller Coulomb interactions. Our ab initio calculations for both compounds are in excellent agreement with photoemission data [3].

[1] M. Aichhorn, L. Pourovskii, V. Vildosola, M. Ferrero, O. Parcollet, T. Miyake, A. Georges, and S. Biermann, Phys. Rev. B 80, 085 101 (2009). [2] L. Vaugier, H. Jiang, and S. Biermann (to be published). [3] C. Martins, M. Aichhorn, L. Vaugier, and S. Biermann, Phys. Rev. Lett. 107, 266 404 (2011).

Contact: [email protected]

75 2012 Electronic Structure Workshop Wake Forest University

Adsorption of Alanine on Ferroelectric Surfaces

M. A. M´endez Polanco, I. Grinberg, and A. M. Rappe

The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, PA 19104–6323, USA.

Chiral purity is of great importance for pharmaceutical and food industries due to poten- tially contrasting biological activity that enantiomers could posses [1, 2]. It has also been important in studies of the origin of life addressing the emergence of biological homochiral- ity [2]. The ultimate link between these and other fields studying enantioselective processes is the importance of chiral adsorption on solids surfaces [3]. Here, using DFT calculations, we study the adsorption of (R,S)-enantiomers of alanine on model ferroelectric (FE) surfaces. We found that there is an enantiospecific adsorption of approximately 80 meV ( 1.84 kcal/mol), about twice the di↵erential energy of adsorption ⇡ found for this aminoacid on chiral calcite [4]. This first study reveals the potential use of FE surfaces for simple, accessible and con- trollable resolution of racemic mixtures.

[1] Z.ˇ Sljivanˇcanin,ˇ K. V. Gothelf, and B. Hammer, J. Am. Chem. Soc. 124, 14789 (2002). [2] S. D. Senanayake and H. Idriss, Proc. Natl. Acad. Sci. USA 103, 1194 (2006). [3] R. M. Hazen and D. S. Sholl, Nature Mater. 2, 367 (2003). [4] A. Asthagiri and R. M. Hazen, Molecular Simulation 33, 343 (2007).

Contact: [email protected]

76 2012 Electronic Structure Workshop Wake Forest University

Reconstruction and Disorder at Compound Semiconductor Surfaces

N. A. Modine,1 J. C. Thomas,2 J. Mirecki Millunchick,2 and A. Van der Ven2

1Sandia National Laboratories, Albuquerque, NM 87185, USA. 2University of Michigan, Ann Arbor, MI 48109, USA.

Compound III-V semiconductor surfaces reconstruct to minimize dangling bonds and sur- face free energy. The specific atomic structure of the reconstruction is determined by a number of competing factors, including local chemical bonding and long range electrostatic and strain interactions. In alloys, these long-range interactions can induce coexistence of dif- ferent reconstructions and can couple surface reconstruction domains to the morphology and compositional non-uniformity of the grown material. At typical growth temperatures, sur- face disorder and the associated configurational and vibrational entropy also have a strong influence on the observed surface structure. In collaboration between the DOE Center for Integrated Nanotechnologies (CINT) and the University of Michigan, we have been devel- oping computational tools to help achieve a quantitative understanding of surface structure and nanostructure formation at semiconductor alloy surfaces. In order to help determine the atomistic structures of experimentally observed surface reconstructions, we have developed an algorithm that systematically enumerates all possible structures of a specified symmetry consistent with a widely applicable set of rules. Building on the resulting structures, we have coupled density functional theory calculations, the cluster expansion approach, and Monte Carlo methods to address the e↵ects of the disorder associated with alloying and finite-temperature on surface reconstruction.

This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Oce of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Contact: [email protected]

77 2012 Electronic Structure Workshop Wake Forest University

Generalized Unitary Bogoliubov Transformation that Breaks Fermion Number Parity

J. E. Moussa

Sandia National Laboratories, Albuquerque, NM 87185, USA.

The standard Bogoliubov transformation is generalized to enable fermion number parity breaking. The new transformation can diagonalize linear-plus-quadratic fermion Hamilto- nians that include a number parity operator to commute with fermion operators. Number parity breaking can be used to model number fluctuations in low-energy fermion modes caused by strong interactions. This e↵ect is compared and contrasted with parity-conserving number fluctuations in the Bardeen-Cooper-Schrie↵er theory of superconductivity.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Contact: [email protected]

78 2012 Electronic Structure Workshop Wake Forest University

Hybrid Density Functional Study of the Electronic Properties for (Hg,Cd)Te Systems

J. W. Nicklas and J. W. Wilkins

Department of Physics, Ohio State University, Columbus, OH 43210, USA.

The HgCdTe alloy is used in high-performance infrared detection applications with a band gap range extending across the infrared spectrum. HgTe in particular has sparked interest for its topological insulating behavior in quantum well devices due to its band inverted nature. We test the quality of the newer hybrid screened functional, HSE, on the two contrasting materials: HgTe (semimetal) and CdTe (semiconductor) to see how well it performs under a range of setupsi [1]. A direct comparison of HSE with the standard DFT functional PBE to experiment for the HgCdTe alloy reveals HSE is able to reproduce the experimental crossover composition of 17% Cd concentration when the alloy goes from a semimetal to semiconductor, whereas PBE overestimates this composition at 67% Cd concentration. HSE also predicts a higher valence band o↵set of 0.53 eV in the HgTe/CdTe heterostructure than previous first-principle and early experimental results, but in good agreement with the more recent experimental results.

Supported by DOE-Basic Energy Science DOE-BES-DMS (DEFG02-99ER45795). Computing resources are provided by NERSC and OSC.

[1] J. W. Nicklas and J. W. Wilkins, Phys. Rev. B 84, 121308(R) (2011).

Contact: [email protected]

79 2012 Electronic Structure Workshop Wake Forest University

Accurate Correlation Energies from a Renormalized Adiabatic Local Density Approximation

T. Olsen and K. S. Thygesen

Center for Atomic-Scale Materials Design, Department of Physics, Technical University of Denmark, DK–2800 Kongens Lyngby, Denmark.

The adiabatic connection fluctuation-dissipation theorem with the random phase approx- imation (RPA) has recently been applied with success to obtain correlation energies of a variety of chemical and solid state systems. The main merit of this approach is the improved description of dispersive forces while chemical bond strengths and absolute correlation en- ergies are systematically underestimated. In this work we extend the RPA by including a parameter-free renormalized version of the adiabatic local density (ALDA) exchange- correlation kernel. The renormalization consists of a (local) truncation of the ALDA kernel for wave vectors q>2kF , which is found to yield excellent results for the homogeneous electron gas. In addition, the kernel significantly improves both the absolute correlation energies and atomization energies of small molecules over RPA and ALDA. The renormal- ization can be straightforwardly applied to other adiabatic local kernels.

This work was supported by The Danish Council for Independent Research through the Sapere Aude program and the Danish Center for Scientific Computing.

Contact: [email protected]

80 2012 Electronic Structure Workshop Wake Forest University

Accurate Computational Studies of Carbon Doped Two-Dimensional Boron-Nitride

H. Park,1 A. Wadehra,1 J. Wilkins,1 and A. C. Neto2

1Department of Physics, The Ohio State University, Columbus, OH 43210, USA. 2Graphene Research Centre, National University of Singapore, Singapore, and Boston University, Boston, MA, USA.

The great success of graphene has vastly extended the range of possible applications of an atomic-layer two-dimensional (2D) crystals with a plethora of new materials. One of these materials is the 2D hexagonal structure of boron nitride (h-BN). h-BN has a wide band gap and a lattice constant similar to that of graphene. We show that even small quantities of C atoms can o↵er new functionalities and transform h-BN to be an amazing playground for 2D physics. Large-scale accurate density-functional-theory calculations with the Heyd-Scuseria- Ernzerhof (HSE) hybrid functional study the electronic and the magnetic properties of h-BN with substitutionally embedded carbon atoms. Results of local magnetic moments induced by substitution and their interactions will be presented for low C concentrations. We will also show the electronic structures of quantum dots made of carbon nano-domains for applications in optics and opto-electronics.

This work was supported by DOE-BES-DMS (DEFG02-99ER45795). We used computational resources of the NERSC, supported by the U.S. DOE (DE-AC02-05CH11231), and the OSC. AHCN acknowledges DOE grant DE-FG02-08ER46512, ONR grant MURI N00014-09-1-1063, and the NRF-CRP award “Novel 2D materials with tailored properties: beyond graphene” (R-144-000-295-281).

Contact: [email protected]

81 2012 Electronic Structure Workshop Wake Forest University

Stability of Surface States of Topological Insulators upon Nonmagnetic Adsorption

K. Park

Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA.

Topological insulators (TIs) draw great attention due to their unique quantum properties and various applications. TIs possess metallic surface states within bulk band gaps induced by strong spin-orbit coupling (SOC). TIs allow the surface states to cross Fermi level only an odd number of times for each surface, which was observed in angle-resolved photoe- mission spectra (ARPES) on Bi-based alloys. Recently, novel phenomena and applications have been proposed for TIs interfaced with other types of materials such as magnetic or superconducting materials. Here we present a systematic study of interface-induced e↵ects in TIs Bi2Te3 and Bi2Se3, using density-functional theory including SOC self-consistently. Surface states are identified using the method discussed in Ref. [1]. Interfaces are simulated by Si adsorption only on top of Bi2Te3 and Bi2Se3 slabs. Even with nonmagnetic adsorption, dispersion of the top surface states is qualitatively a↵ected by momentum-dependent coupling to bulk-like states, Si-dominated states, or Rashba-split quantum-well states, which depends on adsorption sites and topological insulator types. Our findings provide insight into engineering new heterostructures using interfaces for devices or discovering new phenomena.

This work was supported by NSF DMR-0804665 and SDSC Trestles under DMR060009N.

[1] K. Park et al., Phys. Rev. Lett. 105, 186801 (2010).

Contact: [email protected]

82 2012 Electronic Structure Workshop Wake Forest University

Semistochastic Projection

F. R. Petruzielo,1 A. A. Holmes,1 H. J. Changlani,1 M. P. Nightingale,2 and C. J. Umrigar1

1Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA. 2Department of Physics, University of Rhode Island, Kingston, RI 02881, USA.

We introduce a semistochastic implementation of the power method to compute the domi- nant eigenvalue and eigenvector of very large matrices. The method is semistochastic in that the matrix multiplication is partially implemented numerically exactly and partially in the sense of expectation value only. Compared to a fully stochastic method, the semistochastic approach significantly reduces the computational time required to obtain the eigenvalue to a specified statistical uncertainty. This is demonstrated by the application of the semistochas- tic quantum Monte Carlo method to a molecular system in a discrete basis representation and to a system with a sign problem, the fermion Hubbard model.

This work was supported in part by NSF CHE-1112097 and DOE-CMCSN DE-SC0006650.

Contact: [email protected]

83 2012 Electronic Structure Workshop Wake Forest University

Valence Band E↵ective Hamiltonians in Nitride Semiconductors from QSGW Band Structures

A. Punya and W. R. L. Lambrecht

Department of Physics, Case Western Reserve University, Cleveland, OH 44106, USA.

To determine the electronic states, optical properties and transport in quantum wells, quan- tum wires and quantum dots of nitride semiconductors, one use valence band e↵ective Hamiltonians within the e↵ective mass approximation. Athough significant experimental and theoretical work has been performed in the past, basic parameteres such as the Rashba Sheka Pikus (RSP) Hamiltonian parameters are still uncertain [1]. In the present work, the electronic band structures of wurtzite AlN, GaN and InN as well as the RSP Hamiltonian parameters are determined by using the quasiparticle self-consistent GW (QSGW) approx- imation [2, 3] in a full-potential linearized mun tin orbital (FP-LMTO) implementation [4]. The present GW implementation o↵ers more accurate band structures beyond the local density approximation(LDA) since the self-energy is calculated a real space represntation and able to interpolate on arbitrarily fine k-point meshes. Hence, splittings and e↵ective masses can be obtained accurately.We find the crystal field splitting in GaN (12 meV) in much closer agreement with experiment than previous work and obtain a negative spin orbit coupling for InN. Moreover, we have generalized the method of invariants to crystals with orthorombic symmetry, such as ZnSiN2 ZnGeN2, ZnSnN2 and CdGeN2 and determined the corresponding Hamiltonian parameters. These materials have two di↵erent crystal field splittings and several additional e↵ective mass parameters. The spin-orbit splitting param- eters in them are found to be negligibly small.

This work is funded by the NSF.

[1] I. Vurgaftman and J. R. Meyer, J. Appl. Phys. 94, 3675 (2003). [2] M. van Schilfgaarde, T. Kotani, and S. Faleev, Phys. Rev. Lett. 96, 226 402 (2006). [3] T. Kotani, M. van Schilfgaarde, and S. V. Faleev, Phys. Rev. B 76, 165 106 (2007). [4] T. Kotani and M. van Schilfgaarde, Phys. Rev. B 81, 125 117 (2010).

Contact: [email protected]

84 2012 Electronic Structure Workshop Wake Forest University

Ecient, Pseudopotential-Free Auxiliary-Field Quantum Monte Carlo Calculations in Solids

W. Purwanto, S. Zhang, and H. Krakauer

Department of Physics, College of William & Mary, Williamsburg, VA 23187, USA.

We present an approach for ecient, pseudopotential-free many-body calculations in peri- odic solids using the phaseless auxiliary-field quantum Monte Carlo (AFQMC) method [1]. We employ the frozen core (FC) technique to obviate the need for pseudopotentials. In parallel to many-body quantum chemistry methods, tightly-bound inner electrons occupy frozen canonical orbitals, which are determined from a lower level of theory, such as Hartree- Fock or CASSCF. Since AFQMC random walks take place in a many-electron Hilbert space spanned by a one-particle basis, FC can be realized without introducing additional ap- proximations. The same formalism also allows a basis transformation (downfolding) to an e↵ective one-particle orbital basis using, for example, a truncated set of Kohn-Sham DFT orbitals. Both FC and downfolding provide significant computational savings over fully correlating all the electrons in full plane-wave basis, while retaining excellent transferabil- ity and accuracy. We demonstrate the approach by calculating the equation of state of crystalline MnO in antiferromagnetic and ferromagnetic phases. Twist-average boundary condition [2] and a finite-size correction [3] are employed to minimize the e↵ect of finite simulation cell. AFQMC in this basis reproduces the results of the full plane-wave basis many-body calculations, and leads to accurate determination of the magnetic ordering.

This work is funded by the DOE, NSF and ONR. Computing is provided by the Oak Ridge Leadership Facility and the Center for Piezoelectrics by Design.

[1] S. Zhang and H. Krakauer, Phys. Rev. Lett. 90, 136401 (2003). [2] C. Lin, F. H. Zong, and D. M. Ceperley, Phys. Rev. E 64, 016702 (2001). [3] F. Ma, S. Zhang, and H. Krakauer, Phys. Rev. B 84, 155130 (2011).

Contact: [email protected]

85 2012 Electronic Structure Workshop Wake Forest University

Formal Valence, d Occupation, and Charge-Order Transitions

Y. Quan,1 V. Pardo,2 and W. Pickett1

1Department of Physics, University of California Davis, Davis, CA 95616, USA. 2Departamento de Fsica Aplicada, Universidade Santiago de Compostela, Spain.

While the formal valence concept has been tremendously important in materials physics, its very loose connection to actual charge leads to diculties in modeling its consequences and interpreting data correctly. We point out, taking several transition metal oxides (La2V CuO6, Y NiO3, CaF eO3, AgNiO2, V4O7) as examples, that while dividing the crys- tal charge into atomic contributions is an ill-posed activity, the 3d occupation of a cation (and more particularly, di↵erences) is readily available in first principles calculations. We discuss these examples, which include distinct charge states and charge-order (or dispro- portionation) systems, where di↵erent charge states of cations have identical 3d orbital occupation. Implications for theoretical modeling of such charge states and charge-ordering transitions are discussed.

Work at UC Davis was supported by DOE grant DE-FG02-04ER46111. V.P. acknowledges support from the Spanish Government through the Ramon y Cajal Program.

Contact: [email protected]

86 2012 Electronic Structure Workshop Wake Forest University

Electronic and Optical Properties of Benzylpiperazine/CuI(111) System

T. Rawal,1 V. Turkowski,1 R. Blair,2 and T. S. Rahman1

1Department of Physics and NSTC, 2Department of Chemistry and Forensic Science Center, University of Central Florida, Orlando, FL 32816, USA.

We present results of analysis of possible mechanisms responsible for the recently observed [1] strong orange fluorescence of Benzylpiperazine (BZP) molecules adsorbed on a CuI thin film. To evaluate the electronic structural properties of CuI(111) film, we carry out first- principles calculations based on Density-Functional Theory with the Generalized-Gradient Approximation using PBE exchange correlation functional along with the Hubbard Param- eter (U ). In particular, we examine the band structure and electronic density of states of the CuI film with the lowest energy, (111), geometry and that with defect states. We also calculate the excited states of the BZP molecule using time dependent density functional theory. We find that while the two parts of the system, CuI slab and BZP molecule, are individually optically in-active in the specific energy window, they produce strong visible light emission when coupled together. We trace the reason for such emission to optical transitions between the excited states of the molecule and the CuI(111) surface states. We discuss possible applications of the e↵ect and that of defect states on other optical properties of the system, including excitonic e↵ects.

This work is funded by DOE, grant No. DE-FG02-07ER46354.

[1] R. Blair, unpublished.

Contact: [email protected]

87 2012 Electronic Structure Workshop Wake Forest University

Graphene Nanoribbons with a Magnetic Edge

W. Y. Ruan,1 Y. Y. Sun,2 S. B. Zhang,2 and M. Y. Chou1,2

1School of Physics, Georgia Institute of Technology, GA 30332, USA. 2Department of Physics, Applied Physics, Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. 3Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.

Analytical and numerical results based on the tight binding model are presented for zigzag graphene nanoribbons with z1 and z1212 edges. We show the crucial importance of the symmetry of the two edges in determining the electronic structures of the system. Examples of significant band gap narrowings due to symmetry breaking are illustrated.

This work is funded by the DOE No. DEFG02-97ER45632.

Contact: [email protected]

88 2012 Electronic Structure Workshop Wake Forest University

Thermodynamic Stability of the CaMnO3 (001) Surface

D. Saldana-Greco, C.-W. Lee, D. Yuan, and A. M. Rappe

The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA.

Relative thermodynamic stability of surface reconstructions, consisting of vacancies, adatoms and additional layers, in both CaO and MnO2 terminations is calculated to predict the sur- face phase diagram of (p2 p2)R45 CaMnO (001) using ab initio thermodynamics. It ⇥ 3 is found that MnO2-based surfaces are dominant within the stability region as temperature is introduced to the secondary phases boundary conditions. The magnetic ordering e↵ects on the surface stability of CaMnO3 are explored leading to significant changes in MnO2 terminated surface phase diagram.

This work is funded by the DOE.

Contact: [email protected]

89 2012 Electronic Structure Workshop Wake Forest University

Transport Properties of Thermoelectric Materials from First Principles

G. Samsonidze,1 B. Kozinsky,1 C.-H. Park,1 J. Garg,2 D. Volja,2 N. Bonini,3 D. Wee,4 M. Fornari,5 and N. Marzari6

1Research and Technology Center North America, Robert Bosch LLC, Cambridge, MA 02142, USA. 2Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 3King’s College, London, UK. 4Ewha Womans University, Seoul, South Korea. 5Central Michigan University, Mount Pleasant, MI 48859, USA. 6Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland.

Thermoelectric e↵ect o↵ers an e↵ective solution for waste heat recovery. Optimization of thermoelectric materials reqiures description of the electrical and thermal transport proper- ties from first principles. Thermal conductivity is governed by the phonon lifetime and it is important to determine the influence of microscopic e↵ects due to alloying, nanostructuring, and grain boundaries on the macroscopic transport coecients of the system. We show how density-functional theory, with the help of the Boltzmann transport formalism and quasi- harmonic analysis, can deliver predictive accuracy in calculating thermal conductivity of single-crystal, alloyed and nanostructured materials. Our method combines first-principles determination of harmonic force constants with the use of model potentials for calculating higher order anharmonic response terms. The latter gives the main contribution to phonon scattering at high temperatures. We validate our scheme by the comparison of the heat conductivity in SiGe alloys with available experimental data. Analysis of the grain size e↵ects is carried out by introducing boundary scattering terms and using Matthiessen rule. We apply this methodology to analyze grain size e↵ects in binary skutterudites CoSb3.We also calculate the electrical conductivity and Seebeck coecient of CoSb3 using a constant electron lifetime approximation. These transport coecients are sensitive to the electronic band structure, and we stress the importance of using the GW quasiparticle energies as opposed to the DFT Kohn-Sham eigenvalues.

Contact: [email protected]

90 2012 Electronic Structure Workshop Wake Forest University

Coupled Cluster Studies of Molecules in Condensed Matter Environments

K. Schwarz,1 J. Yang,2 R. Sundaraman,3 J. Lee,3 G. Chan,2 and T. A. Arias3

1Department of Chemistry, Cornell University, Ithaca, NY 14853, USA. 2Department of Chemistry, Princeton University, Princeton, NJ 08540, USA. 3Department of Physics, Cornell University, Ithaca, NY 14853, USA.

Coupled cluster is a highly accurate quantum chemistry method, but the computational costs associated with coupled cluster scale poorly with the number of included electrons. Methods to reduce the required number of included electrons in such calculations [1] can help reduce the costs associated with these calculations. Through the use of external potentials from lower-level calculations, we have performed coupled cluster calculations on molecules surrounded by a variety of environments. We have implemented and tested this approach with the DALTON quantum chemistry code and the JDFTx density functional theory code. Our method is applicable to molecules in environments that can be described by potentials, with diverse applications including solvation studies and the refinement of eigenvectors of the molecular response matrix.

[1] K. Schwarz, R. Sundararaman, K. Letchworth-Weaver, T. A. Arias, and R. G. Hennig, Phys. Rev. B 85, 201 102(R) (2012).

Contact: [email protected]

91 2012 Electronic Structure Workshop Wake Forest University

Self-Di↵usion of Small Ag, Cu, and Ni Islands on fcc(111) Surfaces: An Application of SLKMC-II

S. I. Shah, G. Nandipati, A. Kara, and T. S. Rahman

Department of Physics, University of Central Florida, FL, USA.

A few years ago, our group proposed a self-learning kinetic Monte Carlo (SLKMC) scheme [1] in which inclusion of a pattern recognition scheme and on-the-fly calculation of activation energy barriers allowed the creation of a data base of possible di↵usion processes for adatom islands, containing up to 30 atoms. While this addressed the issue of lack of completeness of di↵usion mechanisms in standard KMC simulations, the scheme was limited to adatom occupancy of only the fcc sites. Here we report the development of a new pattern-recognition scheme that takes into account both fcc and hcp adsorption sites in performing self-learning kinetic Monte Carlo (SLKMC-II) simulations on fcc(111) surfaces. In this scheme, the local environment of each under-coordinated atom is uniquely identified by grouping the fcc and hcp sites and the top-layer substrate atoms around it into hexagonal rings. As simulation progresses, all possible processes—including shearing, reptation and concerted gliding, which may involve fcc-fcc, hcp-hcp and/or fcc-hcp moves are automatically found, and their energetics calculated on the fly. We present results of application of this new pattern-recognition scheme to the self-di↵usion of (Mg) on M(111), where M = Cu, Ag or Ni.

This work was supported by DOE-BES under grant DE-FG02-07ER46354.

[1] O. Trushin, A. Karim, A. Kara , and T. S. Rahman, Phys. Rev. B 72, 115401 (2005).

Contact: [email protected]

92 2012 Electronic Structure Workshop Wake Forest University

Screened Coulomb and Exchange Parameters of Localized Electrons in the Self-Consistent Constrained RPA

B.-C. Shih and P. Zhang

Department of Physics, University at Bu↵alo, State University of New York, Bu↵alo, NY 14260, USA.

Screened Coulomb and exchange interactions play an important role in studying the struc- tural and electronic properties of strongly correlated materials. The strong Coulomb in- teraction between localized electrons have prompted the theoretical electronic-structure method beyond the traditional density functional theory approximation method. For these strongly correlated systems, it has been recognized that the localized electrons are better described in the local basis where their Coulomb interactions are generally parameterized. Evaluating these parameters from ab initio approaches can provide better understanding for the strong-correlation e↵ects. We have developed a self-consistent first-principles approach [1] to evaluate the screened Coulomb and exchange parameters using the constrained random phase approximation [2] (cRPA). In the implementation, the localized electrons are described in terms of the maximally localized Wannier functions and the screening is evaluated by direct calculation of the dielectric function. We present a comprehensive study of the self-consistent cRPA approach and the calculated parameters for the 3d electrons in the transition metals and transition metal oxides. An extended application of the cRPA approach is to evaluate the screened Coulomb interactions for localized defect states. Preliminary results for defects systems are also presented.

This work is funded by the NSF under Grant No. DMR-0946404.

[1] B. Shih, Y. Zhang, W. Zhang, and P. Zhang, Phys. Rev. B 85, 045132 (2012). [2] F. Aryasetiawan, M. Imada, A. Georges, G. Kotliar, S. Biermann, and A. I. Lichten- stein, Phys. Rev. B 70, 195104 (2004).

Contact: bshih@bu↵alo.edu

93 2012 Electronic Structure Workshop Wake Forest University

Molecular Dynamics Study of Ripples in Graphene and Bilayer Graphene

A. Singh and R. G. Hennig

Department of Materials Science and Engineering, Cornell University, Ithaca, NY, USA.

Transmission electron microscopy experiments have shown that suspended graphene is not perfectly flat, but displays ripples such that the surface normal of graphene varies by sev- eral degrees [1, 2]. For multi-layered graphene, the ripples are suppressed with increasing numbers of layers. Recent experiments demonstrated that ripples in suspended graphene can also be controlled by mechanical and thermally induced strain [3]. Knowledge of and control over the ripples in graphene is desirable for fabricating and designing of strain-based devices. We show using molecular dynamics simulation that thermally induced ripples in suspended single and multi-layer graphene at room temperature result in deviations of the local surface normal by 7 and 4 over regions of 3A˚ for single and bilayer graphene, ± ± respectively. These angular deviations are in excellent agreement with transmission electron microscopy results [2] and confirm that these ripples can be dynamic in nature. We find scaling relationships for the average angular deviations as a function of size of the graphene layer L and the size of the region over which the angle is averaged R, corresponding to the spot size in transmission electron microscopy. For single layer the average angle scales with R/L and for bilayer it scales as R/L5/4. The scaling relations show that, for the same spot size, the measured angular deviations are larger for larger graphene or bilayer graphene sheets. We also study how these angles change as a function of temperature, applied tensile and compressive strain and number of layers in n-layered graphene.

[1] J. C. Meyer et al., Solid State Commun. 143, 101 (2007). [2] J, C. Meyer et al., Nature 446, 60 (2007). [3] W. Bao et al., Nat. Nanotechnol., 4, 562 (2009).

Contact: [email protected]

94 2012 Electronic Structure Workshop Wake Forest University

A Framework to Generalize Polarizable Continuum Models to Capture the Non-Local Dielectric Response of Solvents

R. Sundararaman,1 J. Lee,1 K. Schwarz,2 K. Letchworth-Weaver,1 and T. A. Arias1

1Department of Physics, Cornell University, Ithaca, NY 14853, USA. 2Department of Chemistry, Cornell University, Ithaca, NY 14853, USA.

Polarizable continuum models [1–3], which replace the solvent by a dielectric cavity, are inexpensive approximations to solvent e↵ects on the electronic structure of solvated systems. Despite the drastic approximation of replacing solvent molecules with a continuum at the atomic scale, these models reasonably capture ground-state solvation energies and to some degree, the solvent shifts in excited state spectra. We present a generalization of the polarizable continuum model derived from a series of approximations within classical density functional theory for the rotational response, and from electronic density functional perturbation theory for the polarization response. This model naturally captures the non-local nature of the solvent dielectric response due to finite molecular size for rotations and the finite extent of the eigenfunctions of the susceptibility. We demonstrate applications of the resulting theory in density functional, coupled cluster and quantum Monte Carlo calculations. Additionally, we find significant improvements in the solvent shifts of excited states predicted by the GW method.

This work is funded by the Energy Materials Center at Cornell (EMC2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Oce of Science, Oce of Basic Energy Sciences under Award Number DE-SC0001086.

[1] J. Tomasi, B. Mennucci, and R. Cammi, Chem. Rev. 105, 2999 (2005). [2] S. A. Petrosyan, A. A. Rigos, and T. A. Arias, J. Phys. Chem. B 109, 15436 (2005). [3] O. Andreussi, I. Dabo, and N. Marzari, J. Chem. Phys. 136, 064102 (2012).

Contact: [email protected]

95 2012 Electronic Structure Workshop Wake Forest University

Bloch-type Ferroelectric Domain Walls in Rhombohedral BaTiO3

M. Taherinejad,1 D. Vanderbilt,1 J. Hlinka,2 and V. Stepkovaand2

1Department of Physics, Rutgers University, Piscataway, NJ 08854, USA. 2Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance, 18221 Praha, Czech Republic.

Ferroelectric domain walls (FDWs) are usually considered to be of Ising type, in which P , the projection of the polarization vector onto the plane of the domain wall, simply reversesk itself by passing through zero along a high-symmetry path as one scans through the domain wall. Ising FDWs tend to be favored because ferroelectrics are generally strongly electrostrictive, so that a rotation of P away from this high-symmetry path would entail a significant elastic energy cost. However,k there have been some theoretical predictions of the presence of Bloch and even N´eel components in some FDWs [1, 2]. Most recently, it has been predicted, in the framework of a phenomenological Ginzburg-Landau-Devonshire (GLD) model, that the 180 FDWs in rhombohedral BaTiO3 are of Bloch type [3]. In the low-temperature rhombohedral phase of BaTiO3, the possible mechanically com- patible and electrically neutral FDWs are of three types: R71, R109, and R180, where the angle denotes the rotation relating P1 and P2 (the polarizations on either side of the wall). The plane of the domain wall can be either 211¯ or 110¯ for the 180 FDW, and { } { } is normal to P1 + P2 for the other two FDWs. We have investigated the R71, R109, and R180 110¯ FDWs in BaTiO using first-principles calculations within the local-density { } 3 approximation (LDA). Our calculations confirm the Bloch nature of the R180 110¯ wall, which can be thought { } of as a combination of Ising R71 and R109 FDWs. Comparison of the first-principles results and the GLD model [3] suggests that a 40% reduction in the gradient term in the GLD model is needed to bring agreement with the first-principles results. The R71 FDW is found to be of Ising type; this is consistent with expectations since the Bloch component for this wall points towards the center of a cube face, which is not one of the preferred directions in the rhombohedral phase. For the R109 FDW, on the other hand, the Bloch component of P does point toward a rhombohedral polarization direction, making the Bloch configuration competitive with the Ising one in this case. In fact, the Bloch R109 FDW can be considered as a combination of two Ising R71 walls, and the energy di↵erence between this and the Ising R109 FDW is only a few meV.

[1] B. Meyer and D. Vanderbilt, Phys. Rev. B 53, R5969 (1996). [2] D. Lee et al., Phys. Rev. B 80, 060102 (2009). [3] P. Marton, I. Rychetsky, and J. Hlinka, Phys. Rev. B 81, 144125 (2010).

Contact: [email protected]

96 2012 Electronic Structure Workshop Wake Forest University

Many-Body Study of Cobalt Adatoms Adsorbed on Graphene

Y. Virgus, W. Purwanto, H. Krakauer, and S. Zhang

Department of Physics, College of William & Mary, Williamsburg, VA 23187, USA.

Since its discovery, graphene has been the subject of intense e↵orts to adapt it for a variety of promising applications. One proposal is to adsorb transition metal atoms to provide localized magnetic moments for use in spintronic devices. Several theoretical [1–3] and ex- perimental [4] studies have examined Co adatoms on graphene. Calculations of Co/graphene systems have largely been done at the density functional theory (DFT) level with local or semi-local functionals, or with an empirical Hubbard on-site repulsion U (LDA+U). How- ever, there has been controversy regarding the bonding nature of Co/graphene systems as these results have shown significant variations with how the correlation e↵ects are treated. We use auxiliary-field quantum Monte Carlo (AFQMC) [5–6] to calculate the binding en- ergy of Co/benzene systems. An embedding size-correction scheme is employed to eciently calculate the binding energy of Co/graphene system as a function of Co absorption height.

This work is funded by the DOE, ONR, and NSF.

[1] T. O. Wehling, A. V. Balatsky, M. I. Katsnelson, A. I. Lichtenstein, and A. Rosch, Phys. Rev. B 81, 115 427 (2010). [2] D. Jacob and G. Kotliar, Phys. Rev. B 82, 085 423 (2010). [3] T. O. Wehling, A. I. Lichtenstein, and M. I. Katsnelson, Phys. Rev. B 84, 235 110 (2011). [4] V. W. Brar et al., Nat. Phys. 7, 43 (2011). [5] S. Zhang and H. Krakauer, Phys. Rev. Lett. 90, 136 401 (2003). [6] W. A. Al-Saidi, S. Zhang, and H. Krakauer, J. Chem. Phys. 124, 224 101 (2006).

Contact: [email protected]

97 2012 Electronic Structure Workshop Wake Forest University

Hybrid Density Functional Study of 2D Graphene-Boron Nitride Nanostructures

A. Wadehra,1 H. Park,1 J. W. Wilkins,1 and A. H. C. Neto2

1Department of Physics, The Ohio State University, Columbus, OH 43220, USA. 2 Graphene Research Centre, National University of Singapore, Singapore and Boston University, Boston, MA, USA.

Graphene has attracted enormous research interest in the last few years because of its in- triguing physics as well as application potential. Recent synthesis of 2D BNC nanostructures by doping graphene with a wide bandgap insulator boron nitride (BN) has unveiled new pos- sibilities for this material [1]. BNC nanostructures are semiconductors and possess interest- ing properties that are distinct from the parent compounds. Reliable theoretical estimates can play a big role by predicting the feasibility and usefulness of still largely unexplored BNC nanostructures, and hence provide a route to engineer their electronic structures. We study electronic structures and magnetic properties of a variety of 2D BNC nanostructures using a hybrid exchange-correlation functional HSE (Heyd-Scuseria-Ernzerhof) in density functional theory (DFT). We show that their properties can be gradually tuned and are sensitive to composition and the type of configurations. In agreement with experimental observation, a strong tendency to phase-segregate exists for low concentration of BN in graphene.

This work is supported by DOE-BES-DMS (DEFG02-99ER45795). We used computational resources of the NERSC, supported by the U.S. DOE (DE-AC02-05CH11231), and the OSC. AHCN acknowledges DOE grant DE-FG02-08ER46512, ONR grant MURI N00014-09-1-1063, and the NRF-CRP award “Novel 2D materials with tailored properties: beyond graphene” (R-144-000-295-281).

[1] L. Ci et al., Nature Materials 9, 430 (2010).

Contact: [email protected]

98 2012 Electronic Structure Workshop Wake Forest University

Electronic Structure of High Pressure Phases TiO2: A Hybrid Functional and GW Method’s Study

B. Wang,1 X. Jiang,1 C. Arhammar,˚ 2 and R. Ahuja1,3

1Department of Material Science and Engineering, Royal Institute of Technology, Stockholm, S-10044, Sweden. 2Sandvik Machining Solutions R&D S-126 80 Stockholm, Sweden. 3Division of Materials Theory, Department of Physics and Astronomy, Uppsala University, S-751 21 Uppsala, Sweden.

Semiconductors with proper band gap can be used as the photoelectrodes for photochemical energy conversion processes. Titanium dioxide has good corrosion resistance in aqueous solutions and is a good candidate for photoelectrodes. The limitation of the anatase phase of TiO2 is its large band gap,which make it unable to absorb sunlight in the visible range. High pressure phases of TiO2 like fluorite and pyrite have optimal band gap given by DFT [1]. In this paper, the electronic properties of high pressure phases of TiO2, flourite, pyrite and cotunnite, have been investigated by hybrid funtional and GW methods. Our calculations suggest that the band gap of fluorite and pyrite are 1.72 eV and 2.34 eV using the hybrid functional HSE06, respectively. The calculated band gap by GW methods is larger than hybrid functional methods, which gives 2.15 eV and 2.45 eV for the fluorite and pyrite phases. For the comparison of the calculated results of anatase phase, we claim that the calculated band gap of fluorite and pyrite phases have the optimal band gap to absorb visible light for photocatalysis to decompose water. The imaginary part of the dielectric function has also been calculated for fluorite, pyrite, and anatase phases using the Bethe-Salpether (BSE) method. The dielectric function calculated by BSE for the anatase phase agrees well with experiment and with previous studies, using the same level of theory. Therefore we expect that we are also able to predict the optical properties of the high pressure phases of TiO2 by the BSE method.

[1] M. Mattesini, J. S. de Almeida, L. Dubrovinsky, N. Dubrovinskaia, B. Johansson, and R. Ahuja, Phys. Rev. B 70, 115101 (2012).

Contact: [email protected]

99 2012 Electronic Structure Workshop Wake Forest University

The Disorder E↵ects in Ru Substituted BaFe2As2: Realization of Superdi↵usion Mechanism

L. Wang,1 T. Berlijn,1 Y. Wang,2 C.-H. Lin,1,3 P. J. Hirschfeld,2 and W. Ku1,3

1Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA. 2Department of Physics, University of Florida, Gainesville, FL 32611, USA. 3Physics Department, State University of New York, Stony Brook, NY 11790, USA.

We investigate the disorder e↵ects in Ru substituted BaFe2As2 using the recently developed Wannier function based e↵ective Hamiltonian method for disordered system [1, 2]. Although the disordered Ru substitutions introduce strong scattering in the Fe bands, the states near the Fermi level are amazingly coherent. This unexpected “protection” of Fermi surfaces against impurity scattering is shown to originate from a strong interference between the correlated on-site and o↵-site impurity e↵ects, a realization of superdi↵usion previously proposed in 1D models. Finally, systematic e↵ects of Ru substitution will be discussed, including the change of carrier density, the enhanced 3D characteristic, and the recent controversy on the doping evolution of the Fermi surfaces.

Work supported by DOE DE-AC02-98CH10886 and DOE CMCSN.

[1] T. Berlijn, D. Volja, and W. Ku, Phys. Rev. Lett. 106, 077005 (2011). [2] W. Ku, T. Berlijn, and C. C. Lee, Phys. Rev. Lett. 104, 216401 (2010).

Contact: [email protected]

100 2012 Electronic Structure Workshop Wake Forest University

Microscopic Origin of the Structural Phase Transitions at the Cr2O3 (0001) Surface

A. L. Wysocki, S. Shi, and K. D. Belashchenko

Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE 68588, USA.

The surface of a Cr3O3 (0001) film epitaxially grown on Cr undergoes an unusual reentrant sequence of structural phase transitions (1 1 p3 p3 1 1). In order to under- ⇥ ! ⇥ ! ⇥ stand the underlying microscopic mechanisms, the structural and magnetic properties of the Cr3O3 (0001) surface are here studied using first-principles electronic structure calculations. Two competing surface Cr sites are identified. The energetics of the surface is described by a configurational Hamiltonian with parameters determined using total energy calcula- tions for several surface supercells. E↵ects of epitaxial strain and magnetic ordering on configurational interaction are also included. The thermodynamics of the system is studied using Monte Carlo simulations. At zero strain the surface undergoes a 1 1 p3 p3 ⇥ ! ⇥ ordering phase transition at T 165K. Tensile epitaxial strain together with antiferro- C ⇠ magnetic ordering drive the system toward strong configurational frustration, suggesting the mechanism for the disordering phase transition at lower temperatures.

This work is funded by the NSF through Nebraska MRSEC.

Contact: [email protected]

101 2012 Electronic Structure Workshop Wake Forest University

Wave Packet Dynamics in Twisted Bilayer Graphene

L. Xian,1 Z. Wang,1,2 and M. Y. Chou1,3

1School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA. 2Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112, USA. 3Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan.

It has been shown recently that high-quality epitaxial graphene (EPG) can be grown on the SiC substrate that exhibits interesting physical properties. In particular, the multi- layer graphene films grown on the C-face show rotational disorder. It is expected that the twisted layers exhibit unique new physics that is distinct from that of either single layer graphene or graphite. In this work, the time-dependent wave-packet propagation in twisted bilayer graphene (TBG) is studied based on a tight-binding model with parameters derived from density functional theory. Our results demonstrate intriguing dynamical behavior of electrons in TBG, which is related to the specific interlayer coupling. By varying the twist angle and the initial wave packet, we can e↵ectively control the propagation of electrons in TBG.

Contact: [email protected]

102 2012 Electronic Structure Workshop Wake Forest University

Optical Phonon Anomaly in Bilayer Graphene with Ultrahigh Carrier Densities

J.-A. Yan,1 K. Varga,2 and M.-Y. Chou3,4

1Department of Physics, Astronomy, and Geosciences, Towson University, Towson, MD 21252, USA. 2Department of Physics and Astronomy, Vanderbilt University, Nashvil le, TN 37035, USA. 3School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA. 4Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.

Electron-phonon coupling (EPC) in bilayer graphene (BLG) at di↵erent doping levels is studied by first-principles calculations. The phonons considered are long-wavelength high- energy symmetric (S) and antisymmetric (AS) optical modes. Both are shown to have distinct EPC-induced phonon linewidths and frequency shifts as a function of the Fermi level EF . We find that the AS mode has a strong coupling with the lowest two conduction bands when the Fermi level EF is nearly 0.5 eV above the neutrality point, giving rise to a 1 1 giant linewidth (more than 100 cm ) and a significant frequency softening (60 cm ). Our ab initio calculations show that the origin of the dramatic change arises from the unusual band structure in BLG. The results highlight the band structure e↵ects on the EPC in BLG in the high carrier density regime.

[1] Jia-An Yan, K. Varga, and M. Y. Chou, http://arxiv.org/abs/1204.1584.

Contact: [email protected]

103 2012 Electronic Structure Workshop Wake Forest University

Optical Properties of Twisted Bilayer Graphene

H. Zhuang and R. G. Hennig

Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14850, USA.

Using density functional theory (DFT) and tight binding (TB) calculations, we study the band structure of bilayer graphene as a function of twist angles and compare the results to spectroscopic measurements. To test the accuracy of di↵erent exchange-correlation func- tionals in DFT, we first compare the band structure with DFT and G0W0 approximation for bilayer graphene systems with di↵erent twist angles. Our DFT results agree with previ- ous DFT calculations for the weak coupling between the two layers and the band structure. Calculations of the quasiparticle dispersion using the G0W0 approximation show that DFT underestimates the Fermi velocity of bilayer graphene and the bandgap away from the K point. Scaling of the DFT band structure by an empirical parameter that depends on the twist angle accounts for most of the di↵erence between the DFT and the G0W0 approxi- mation. Based on the G0W0 calculations, we test a set of tight-binding parameters for the interlayer coupling Hamiltonian. Using this tight-binding model and DFT we study the optical matrix elements Mop for the intralayer transition and parallel band transition. Our results reveal that parallel band transition plays an important role in the observed G band resonance in Raman spectroscopy for specific twist angles [1]. We also study the electron- phonon coupling strength using the deformation potential approximation. Combining opti- cal matrix elements with the electron-phonon coupling strength, the Raman intensity curves for various twist bilayer graphene are calculated. The results of these Raman line shapes are consistent with experiment.

This work is funded by the NSF through the CCMR. This work used computational resources provided by the NSF through the XSEDE and the TACC.

[1] R. W. Havener, H. Zhuang, L. Brown, R. G. Hennig, and J. Park, Nano Letters, submitted.

Contact: [email protected]

104 4 List of Conference Participants

Addagarla, Tejas Bernholc, Jerry University of Massachusetts at Amherst NC State University [email protected] [email protected]

Aminpour, Maral Birol, Turan University of Central Florida Cornell University [email protected] [email protected]

Baroni, Stefano Bowler, David SISSA, Trieste, Italy University College London, England [email protected] [email protected]

Bartlett, Rodney J. Cancio, Antonio University of Florida Ball State University [email protected]fl.edu [email protected]

Beck, David Canepa, Pieremanuele Princeton University Wake Forest University [email protected] [email protected]

Bennett, Joe Ceperly, David Rutgers University University of Illinois at Urbana-Champaign [email protected] [email protected]

Berlijn, Tom Chan, Garnet Brookhaven National Laboratory Princeton University [email protected] [email protected]

105 2012 Electronic Structure Workshop Wake Forest University

Cheiwchanchamnangij, Tawinan Filippi, Claudia Case Western Reserve University University of Twente, The Netherlands [email protected] c.fi[email protected]

Chen, Zuojing Garrity, Kevin University of Massachusetts at Amherst Rutgers University [email protected] [email protected]

Cho, Kyeongjae Gavin, Brendan University of Texas at Dallas University of Massachusetts at Amherst [email protected] [email protected]

Cho, Sam Georgescu, Alexandru Bogdan Wake Forest University Yale University [email protected] [email protected]

Chou, Mei-Yin Ghosh, Saurabh Academia Sinica Cornell University [email protected] [email protected]

Clark, Bryan Goldsmith, Zachary Princeton University University of Pennsylvania [email protected] [email protected]

Das, Hena G¨orling, Andreas Cornell University University of Erlangen-N¨urnberg, Germany [email protected] [email protected]

Dogan, Mehmet Guo, Yuzheng Yale University Cambridge University [email protected] [email protected]

Dong, Rui Han, Chengbo NC State University Center for High Performance Simulation [email protected] [email protected]

Fellinger, Michael Hatcher, Ryan The Ohio State University Lockheed Martin [email protected] [email protected]

106 2012 Electronic Structure Workshop Wake Forest University

Hodak, Miroslav Jiang, Jie NC State University Yale University [email protected] [email protected]

Holmes, Adam Jiang, Lai Cornell University University of Pennsylvania [email protected] [email protected]

Holzwarth, Natalie Johnson, Daniel Wake Forest University Wake Forest University [email protected] [email protected]

Hong, Jiawang Jurchescu, Oana Rutgers University Wake Forest University [email protected] [email protected]

Hong, Sampyo Kabir, Alamgir University of Central Florida University of Central Florida [email protected] [email protected]

Honrao, Shreyas Kakekhani, Arvin Cornell University Yale University [email protected] [email protected]

Huang, Ling-Yi Kara, Abdelkader Case Western Reserve University University of Central Florida [email protected] [email protected]

Huang, Shouting Kaur, Amandeep Washington University in St. Louis University of California at Davis [email protected] [email protected]

Irikura, Karl Kent, Paul NIST Oak Ridge National Laboratory [email protected] [email protected]

Ismail-Beigi, Sohrab Kestyn, James Yale University The University of Massachusetts at Amherst [email protected] [email protected]

107 2012 Electronic Structure Workshop Wake Forest University

Kim, Jeongnim Liu, Shi Oak Ridge National Laboratory University of Pennsylvania [email protected] [email protected]

Kim, Seungchul Liu, Yuanyue University of Pennsylvania Rice University [email protected] [email protected]

Kolb, Brian Lopez, Graham Wake Forest University Wake Forest University [email protected] [email protected]

Kozinsky, Boris Lu, Deyu Bosch Research Brookhaven National Lab [email protected] [email protected]

Lee, Jeehye Lu, Wenchang Cornell University NC State University [email protected] [email protected]

Lepley, Nicholas Marom, Noa Wake Forest University UT Austin [email protected] [email protected]

Letchworth-Weaver, Kendra Martins, Cyril Cornell University CEA, France [email protected] [email protected]

Li, Qi Martirez, John Mark Wake Forest University University of Pennsylvania [email protected] [email protected]

Li, Yan Marzari, Nicola NC State University EPFL, Switzerland [email protected] nicola.marzari@epfl.ch

Liang, Yufeng Matthews, Rick Washington University in St. Louis Wake Forest University [email protected] [email protected]

108 2012 Electronic Structure Workshop Wake Forest University

McMahon, Je↵ Park, Hyoungki University of Illinois The Ohio State University [email protected] [email protected]

McMinis, Jeremy Park, Kyungwha University of Illinois Virginia Tech [email protected] [email protected]

Mendez Polanco, Miguel Angel Punya, Atchara University of Pennsylvania Case Western Reserve University [email protected] [email protected]

Merle, John Purwanto, Wirawan Winston Salem State University College of William & Mary [email protected] [email protected]

Modine, Normand Quan, Yundi Sandia National Laboratories University of California at Davis [email protected] [email protected]

Montgomery, David Quina, Frank Wake Forest University University of Sao Paulo [email protected] [email protected]

Moussa, Jonathan Ranjan, Vivek Sandia National Laboratories NC State University [email protected] [email protected]

Nicklas, Jeremy Rappe, Andrew The Ohio State University University of Pennsylvania [email protected] [email protected]

Olsen, Thomas Rawal, Takat Technical University of Denmark University of Central Florida [email protected] [email protected]

Parker, James Resta, Ra↵aele U.S. Army Research Oce SISSA, Trieste, Italy [email protected] [email protected]

109 2012 Electronic Structure Workshop Wake Forest University

Ruan, Wen Ying Singh, Arunima Georgia Institute of Technology Cornell University [email protected] [email protected]

Salam, Akbar Soluyanov, Alexey Wake Forest University Rutgers University [email protected] [email protected]

Saldana-Greco, Diomedes Sundararaman, Ravishankar University of Pennsylvania Cornell University [email protected] [email protected]

Samsonidze, Georgy Taherinejad, Maryam Bosch Research Rutgers University [email protected] [email protected]

Schwarz, Kathleen Takenaka, Hiroyuki Cornell University University of Pennsylvania [email protected] [email protected]

Shah, Syed Islamuddin Tan, Bikan University Of Central Florida NC State University [email protected] [email protected]

Shepherd, James Thonhauser, Timo University of Cambridge Wake Forest University [email protected] [email protected]

Sheppard, Daniel Torrent, Marc LANL CEA, France [email protected] [email protected]

Shih, Bi-Ching Tubman, Norm University at Bu↵alo University of Illinois bshih@bu↵alo.edu [email protected]

Shi, Jing Umrigar, Cyrus University of Pennsylvania Cornell University [email protected] [email protected]

110 2012 Electronic Structure Workshop Wake Forest University

Vanderbilt, David Wysocki, Aleksander Rutgers University Cornell University [email protected] [email protected]

Virgus, Yudistira Xian, Lede College of William & Mary Georgia Tech [email protected] [email protected]

Volja, Dmitri Yan, Jia-An University of Pennsylvania Towson University [email protected] [email protected]

Wadehra, Amita Young, Steve The Ohio State University University of Pennsylvania amita@pacific.mps.ohio-state.edu [email protected]

Walter, Eric Yusufaly, Tahir College of William & Mary Rutgers University [email protected] [email protected]

Wang, Cai-Zhuang Zhang, Shiwei Ames Lab, Iowa State University College of William & Mary [email protected] [email protected]

Wang, Limin Zheng, Fan Brookhaven National Laboratory University of Pennsylvania [email protected] [email protected]

Watson, Mark Zhuang, Houlong Princeton University Cornell University [email protected] [email protected]

Wentzcovitch, Renata Zolghadr, Sina University of Minnesota Wake Forest University [email protected] [email protected]

Wright, Alan Zwanziger, Josef Sandia National Laboratories Dalhousie University, Canada [email protected] [email protected]

111 112 5 Index

5.1 Index of Invited Talks

Baroni: Ab initio colors ...... 10 Bartlett: DFT consistency ...... 11 Bennett: Design of multifunctional materials ...... 12 Bowler: Progress in linear scaling DFT with CONQUEST ...... 13 Canepa: Di↵usion of molecules in MOFs ...... 14 Chan: Density matrix entanglement ...... 15 Clark: Approaching strongly correlated systems ...... 16 Filippi: Size-extensive wave functions for QMC ...... 17 G¨orling: Correlation functionals via the random phase approximation ...... 18 Ismail-Beigi: Luttinger-Ward approaches for going beyond DFT ...... 19 Jiang: Topology-based charge partitioning and oxidation states ...... 20 Jurchescu: Crystalline order in organic devices ...... 21 Marom: Dye-sensitized TiO2 clusters from G0W0 ...... 22 Marzari: DFT: Time to move on? ...... 23 McMahon: Molecular dissociation of hydrogen ...... 24 Rappe: Dirac semimetal in three dimensions ...... 25 Resta: Topological order in electronic wavefunctions ...... 26 Sheppard: Solid-state nudged elastic band method ...... 27 Soluyanov: First-principles calculation of topological invariants ...... 28 Torrent: Dense hydrogen by path-integral molecular dynamics ...... 29 Wang: Material prediction ...... 30 Wentzcovitch: Spin crossover systems in the deep mantle ...... 31 Zhang: Auxiliary-field quantum Monte Carlo ...... 32 Zwanziger: Electric and magnetic fields in solids ...... 33

113 2012 Electronic Structure Workshop Wake Forest University

5.2 Index of Posters

Aminpour: The role of van der Waals interaction ...... 36 Berlijn: Disorder substitutions and vacancies in Fe based superconductors ...... 37 Birol: Structural and electronic trends in rutile compounds ...... 38 Cancio: Laplacian-based models for the exchange energy ...... 39 Cheiwchanchamnangij: QSGW calculation of monolayer, bilayer, and bulk MoS2 . . 40 Chen: Spectral propagation schemes for TDDFT ...... 41 Cho: Voltage limit and crystal field splitting in Li-ion batteries ...... 42 Das: Properties of hexagonal rare-earth ferrites ...... 43 Fellinger: Interatomic potentials for bcc metals ...... 44 Garrity: Modeling piezoelectricity in improper ferroelectrics ...... 45 Gavin: Nonlinear eigensolver-based alternative to SCF methods ...... 46 Ghosh: Graphene: reversible spin manipulator of molecular magnet ...... 47 Goldsmith: An improved OPIUM Ce pseudopotential ...... 48 Gonz´alez-Hern´andez: Interface formation of scandium nitride on GaN(0001) . . . . 49 Guo: Metal-insulator transition of V2O3 ...... 50 Holzwarth: Li superionic conductors ...... 51 Hong: Spin-phonon coupling in perovskites ...... 52 Hong: Methanol oxidation on Au/TiO2⇤ ...... 53 Huang: CsSnX3 Band structure calculations by the QSGW method ...... 54 Huang: Lattice vibrational modes in core-shell NWs ...... 55 Jiang: Aminoethanethiol coated CdSe quantum dots ...... 56 Kabir: Transition Metal Nanoparticles: DFT+DMFT ...... 57 Kakekhani: NOx decomposition ...... 58 Kara: Acenes on metal surfaces ...... 59 Kaur: Core polarization e↵ects ...... 60 Kestyn: Real-space all-electron band structure calculations ...... 61 Kim: Work functions of oxides ...... 62 Kolb: Machine learning with network functions ...... 63 Kozinsky: Screening of materials for Li/air batteries ...... 64 Lee: Two-level states in mixed crystal ...... 65 Lepley: Modeling lithium thiophosphates ...... 66 Letchworth-Weaver: Solvated surface studies within joint DFT ...... 67 Liang: Gated bilayer graphene ...... 68 Li: Transport of hot electrons ...... 69 Li: Iodine vacancies in SrI2 ...... 70 Liu: Edges of low-dimensional materials ...... 71 Lopez: Ab initio spin-spin coupling ...... 72 L´opez-P´erez: Properties of Zr adsorption on AIN ...... 73 Lu: Evaluation of model exchange-correlation kernels ...... 74 Martins: Correlations in 4d and 5d transition metal oxides ...... 75 M´endez Polanco: Adsorption of alanine on ferroelectric surfaces ...... 76 Modine: Reconstruction and disorder ...... 77

114 2012 Electronic Structure Workshop Wake Forest University

Moussa: Generalized unitary Bogoliubov transformation ...... 78 Nicklas: DFT study of HgCdTe systems ...... 79 Olsen: Accurate correlation energies from a renormalized ALDA ...... 80 Park: Electronic structures of carbon clusters embedded in graphene ...... 81 Park: Topological insulators upon adsorption ...... 82 Petruzielo: Semistochastic projection ...... 83 Punya: Valence band e↵ective Hamiltonians in nitride semiconductors ...... 84 Purwanto: Ecient, pseudopotential-free AFQMC in solids ...... 85 Quan: D occupation and charge-order transitions ...... 86 Rawal: Benzylpiperazine/CuI(111) ...... 87 Ruan: Graphene nanoribbons ...... 88 Saldana-Greco: Thermodynamic stability of the CaMnO3 (001) surface ...... 89 Samsonidze: Transport properties of thermoelectric materials ...... 90 Schwarz: Coupled cluster studies of molecules ...... 91 Shah: Self-di↵usion of Ag, Cu, and Ni islands on surfaces ...... 92 Shih: Screened Coulomb and exchange parameters of localized electrons ...... 93 Singh: Ripples in graphene and bilayer graphene ...... 94 Sundararaman: Non-local polarizable continuum models ...... 95 Taherinejad: Ferroelectric domain wall in BaTiO3 ...... 96 Virgus: Many-body study of Co on graphene ...... 97 Wadehra: 2D BNC nanostructures using hybrid DFT ...... 98 Wang: High pressure TiO2 ...... 99 Wang: Disorder e↵ects in Ru substituted BaFe2As2 ...... 100 Wysocki: Phase transition at the Cr2O3 surface ...... 101 Xian: Wave packet dynamics in TBG ...... 102 Yan: Optical phonon anomaly in bilayer graphene ...... 103 Zhuang: Optical properties of twisted bilayer graphene ...... 104

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