ANL/ALCF/ESP-17/5 First-Principles Simulations of Functional Materials for Energy Conversion Technical Report for the ALCF Theta Early Science Program Argonne Leadership Computing Facility ALCF Early Science Program (ESP) Technical Report ESP Technical Reports describe the code development, porting, and optimization done in preparing an ESP project's application code(s) for the next generation ALCF computer system. This report is for a project in the Theta ESP, preparing for the ALCF Theta computer system. About Argonne National Laboratory Argonne is a U.S. Department of Energy laboratory managed by UChicago Argonne, LLC under contract DE-AC02-06CH11357. The Laboratory's main facility is outside Chicago, at 9700 South Cass Avenue, Argonne, Illinois 60439. For information about Argonne and its pioneering science and technology programs, see www.anl.gov. 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The views and opinions of document authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, Argonne National Laboratory, or UChicago Argonne, LLC. ANL/ALCF/ESP-17/5 First-Principles Simulations of Functional Materials for Energy Conversion Technical Report for the ALCF Theta Early Science Program edited by Timothy J. Williams and Ramesh Balakrishnan Argonne Leadership Computing Facility prepared by Huihuo Zheng, Christopher Knight, Marco Govoni, Giulia Galli, and Francois Gygi September 2017 This work was supported in part by the Office of Science, U.S. Department of Energy, under Contract DE-AC02-06CH11357. First-Principles Simulations of Functional Materials for Energy Conversion Huihuo Zheng∗1, Christopher Knighty1, Marco Govoniz2, 3, Giulia Gallix2, 3, and Francois Gygi{4 1 Leadership Computing Facility, Argonne National Laboratory, Argonne, IL 2 Institute for Molecular Engineering and Materials Science Division, Argonne National Laboratory, Argonne, IL 3 Institute for Molecular Engineering, University of Chicago, Chicago, IL 4 Department of Computer Science, University of California, Davis, Davis, CA November 19, 2018 1 Introduction Computational modeling has become a very effective approach in predicting properties of materials, and in designing functional materials with targeted structural, thermal, or optical properties. Many electronic structure methods have been developed, including density functional theory (DFT) (see [1] and the references therein), many DFT-based methods [2, 3], quantum Monte Carlo [4], and quantum chemistry methods [5]. Among them, DFT has been widely used in physics and materials science because it is computationally cheaper than other methods but still gives desired accuracy. It also provides a good starting point for higher levels of theory, such as many-body perturbation theory and quantum Monte Carlo. In parallel to these electronic structure method developments, a dramatic increase in computing capabilities over the last decade has enabled large-scale electronic structure calculations to address leading-edge materials science problems. In particular, with Theta at the Argonne Leadership Computing Facility (ALCF), our early science project investigated large-scale nanostructured ma- terials for energy conversion and storage using two open-source electronic structure codes Qbox (http://qboxcode.org) and WEST (http://west-code.org). Qbox is an ab-initio molecular dynamics code based on plane wave DFT, and WEST is a post-DFT code for excited state calculations within many-body perturbation theory. Theta is a Cray/Intel system based on 2nd generation Xeon-Phi processors (code-named Knights Landing (KNL)) and serves as a bridge between the current flagship machine, Mira, a 10 petaflop IBM Blue Gene Q, and the upcoming ALCF-3 Aurora flagship machine. Theta has several distinct ∗[email protected] [email protected] [email protected] [email protected] {[email protected] 1 architectural features, such as Advanced Vector Extensions (AVX-512) and a memory hierarchy, which are expected to enable scientific codes to achieve higher performance. Other key differences between Theta and Mira are the networks (IBM's 5D torus vs. Cray Aries interconnect with 3-level Dragonfly topology) and the overall balance of compute/memory capabilities relative to network performance. It is crucial to understand the combined impact of these technologies on an application's performance (with production quality inputs) in order to scale efficiently to large fractions of the machine and improve the time-to-solution of science-critical calculations. In this report, we summarize our efforts to improve performance of both Qbox and WEST on Theta, some of which have also resulted in improved performance on Mira. We expect the optimizations accomplished during the Theta Early Science project will play a key role in refining the road-map for software development efforts targeting future large-scale computing resources (e.g. Aurora at ALCF). The contents of this report are organized as follows. In Section 2, we give a brief summary of the science objectives explored in this early science project. In Section 3, we introduce the Qbox and WEST application codes and briefly discuss the underlying theories and algorithms { plane wave DFT and many-body perturbation theory. Then, in Section 4, the optimization efforts completed for both codes are discussed. Finally, in Section 5, we comment on the portability of the two codes, in particular highlighting the possibility for workflows to utilize both Mira and Theta as enabled by this project. 2 Science Summary In this project, the electronic properties of several nanostructured materials were investigated for their use in solar and thermal energy conversion devices. In particular, we focused on the opto- electronic properties of inorganic nanostructured samples for use in third generation solar cells, i.e. solar cells exploiting intermediate band transitions and/or multi-exciton generation for carrier mul- tiplication. The goal is to use advanced theoretical methods to improve predictions of the properties of new energy materials thereby accelerating discovery of materials for use in future devices and helping to optimize device performance. We focus on materials exhibiting complex structures on multiple length scales and predict their electronic and thermal properties using a theoretical framework that combines ab initio molecular dynamics with accurate electronic structure methods. The molecular simulations are employed to compute ensemble averages of thermodynamic and transport properties of nanostructured materials with complex morphologies and compositions, inclusive of interfaces between nano- and meso-scale building blocks (see Fig. 1). The same atomistic trajectories subsequently serve as input to many- body perturbation theory calculations to compute electronic properties, including band gaps, band edges, absorption spectra, and dielectric properties. We collaborate with experimental groups (V. Klimov at LANL and D. Talapin at UoC) to determine initial structure models to be used as input for ab initio molecular dynamics simulations for further structural optimization, and subsequently many-body perturbation calculations to determine accu- rately the optoelectronic properties. We focus on group IV and III-V nanoparticles with a variety of ligands, which are currently being tested experimentally. In addition, we compute band gaps and energy level alignment between ligands and constituent nanoparticles (NPs), or between the two constituents in a heterogeneous interface system, so as to optimize both quantities to increase the system hole and electron mobilities. Together, these important microscopic pieces of information 2 (a) (b) (c) (d) (e) Figure 1: Examples of systems investigated in this project using the Qbox and WEST codes on Theta: (a) Silicon nanoparticle; (b) CdSe nanoparticle; (c) PbSe nanoparticle; (d) Pb