Ψk Scientific Highlight Of The Month No. 124 August 2014 Density Functional Theory: Past, present, ... future? R. O. Jones1 Peter-Gr¨unberg-Institut PGI-1 and German Research School for Simulation Sciences, Forschungszentrum J¨ulich, D-52425 J¨ulich, Germany. Abstract In little more than 20 years, the number of applications of the density functional (DF) formalism in chemistry and materials science has grown in astonishing fashion. The number of publications alone shows that DF calculations are a huge success story, and many younger colleagues are surprised to learn that the real breakthrough of density functional methods, particularly in chemistry, began only after 1990. This is indeed unexpected, because the origins are usually traced to the papers of Hohenberg, Kohn, and Sham more than a quarter of a century earlier. Olle Gunnarsson and I reviewed the DF formalism, its applications, and prospects for this journal as we saw them in 1989, and I argued shortly afterwards in Angewandte Chemie that combining such calculations with molecular dynamics should lead to an efficient way of finding molecular structures. Here I take that time (1990) as fixed point, review the development of density related methods back to the early years of quantum mechanics and follow their breakthrough after 1990. The two examples from biochemistry and materials science are among the many current applications that were simply beyond our dreams in 1990. I discuss the reasons why - after two decades of rapid expansion - some of the best-known practitioners in the field are concerned about its future. [email protected] Density Functional Theory: Past, present, . future? R. O. Jones∗ Peter-Gr¨unberg-Institut PGI-1 and German Research School for Simulation Sciences, Forschungszentrum J¨ulich, D-52425 J¨ulich, Germany. (Dated: August 15, 2014) In little more than 20 years, the number of applications of the density functional (DF) formalism in chemistry and materials science has grown in astonishing fashion. The number of publications alone shows that DF calculations are a huge success story, and many younger colleagues are surprised to learn that the real breakthrough of density functional methods, particularly in chemistry, began only after 1990. This is indeed un- expected, because the origins are usually traced to the papers of Hohenberg, Kohn, and Sham more than a quarter of a century earlier. Olle Gunnarsson and I reviewed the DF formalism, its applications, and prospects for this journal as we saw them in 1989, and I argued shortly afterwards in Angewandte Chemie that combining such calculations with molecular dynamics should lead to an efficient way of finding molecular structures. Here I take that time (1990) as fixed point, review the development of density related meth- ods back to the early years of quantum mechanics and follow their breakthrough after 1990. The two examples from biochemistry and materials science are among the many current applications that were simply beyond our dreams in 1990. I discuss the reasons why – after two decades of rapid expansion – some of the best-known practitioners in the field are concerned about its future. CONTENTS VIII. Acknowledgments 18 I. Introduction 1 A.ADIABATICCOUPLING 18 II. The density as basic variable 2 References 19 III. An“approximatepracticalmethod” 4 IV. Density functional formalism 6 I. INTRODUCTION A. Single-particle description of a many-electron system. 6 B. Exchange-correlation energy Exc 7 The density functional (DF) formalism shows that C. Exchange-correlation hole and Exc 8 ground state properties of a system of electrons in an V. DF theory to 1990 9 external field can be determined from a knowledge of A. Condensed matter 9 the electron density distribution n(r) alone. Thomas B. Chemistry 9 (Thomas, 1927) and Fermi (Fermi, 1928) recognized the C. Menton, 1991 10 basic nature of the electron density, and Dirac (Dirac, D. Progress to 1990 11 1930) noted already in 1930 that the state of an atom is VI.Afterthebreakthrough 12 determined completely by its density; one does not need A. Progress and problems 12 to know the much more complicated many-electron wave E 1. Approximations for xc 12 function. This remarkable result was found in the con- 2. Dispersion interactions 13 3. “Strongly correlated” systems 14 text of “self-consistent field” (Hartree-Fock) theory. 4. Developments related to QMC 14 My main focus here is on a property for which DF cal- 5. Notation 15 culations are particularly valuable in chemistry and ma- B. Two applications 15 terials science: the total energy E of a system of electrons C. Other systems 16 1. Classical density functional theory 16 in the presence of ions located at RI. Accurate calcula- 2. Nuclei 16 tions of the entire energy surface E(RI) are possible with traditional (wave function based) methods only for sys- VII. Summary and outlook 17 A. An“approximatepracticalmethod” 17 tems with very few atoms, and E(RI) generally has vast B. Exchange-correlation approximations 17 numbers of maxima and minima at unknown locations. C. Quo vadis? 18 The lowest energy, however, corresponds to the ground state structure, and paths between minima are essential to our studies of chemical reactions, including their acti- vation energies. The observation of Francis Crick in his ∗ Electronic address: [email protected] autobiography (Crick, 1988): 2 “If you want to study function, study struc- 20000 ture.” 1000 may be self-evident to anyone interested in biology or 15000 molecules in general, but it is true in other areas. The 500 DF approach allows us to calculate E(RI), and hence the structure and many related properties, without using experimental input. 10000 Olle Gunnarsson and I reviewed the DF formalism, its 0 history, and its prospects in 1989 (Jones and Gunnars- 1985 1990 son, 1989), and I took a similar perspective in numerous 5000 seminars. A more careful reading of the original liter- Publications / Year ature some years ago caused me to change the format, and I trace DF history here from the first years after the development of quantum mechanics. It is a fascinating 0 story with many players, and I quote in several places 1980 1990 2000 2010 from the original texts. Year Density functional calculations are well established FIG. 1 (Color online) Number of publications per year (1975- now in condensed matter physics and chemistry, but they 2013) on topics “density functional” or “DFT”, according to did not (and do not) find universal acceptance. The Web of Knowledge (July 2014). Inset shows data near 1990 choice of 1990 as fixed point coincides with the publi- on an expanded scale (Mavropoulos, 2014). cation of the review (Jones and Gunnarsson, 1989) and my article in Angewandte Chemie (Jones, 1991), where I advocated DF calculations in the context of molecular I have several goals in writing this review. The huge studies, particularly when combined with molecular dy- growth in the DF literature is only possible if many new- namics (Car and Parrinello, 1985). As shown in Fig. 1 comers are entering the field. This review is written for (Mavropoulos, 2014), it also marks the dramatic increase them and for those in other areas of science who are cu- in the number of publications on density functional top- rious about the DF world. I hope that all share my fasci- 1 ics in recent years. Figure 1 shows clearly two other nation with the formalism and its history. However, per- features: there were three times as many publications in sonal contacts over the years have shown that not only 1991 than in 1990, and there was almost no published newcomers are unfamiliar with the past or the reaction of work claiming to be “density functional” or “DFT” be- different scientific communities as the theory developed. forehand. In 1985, twenty years after its modern formula- Density related methods are also important in other ar- tion and in the year that the combined DF/MD approach eas, including classical systems and nuclei, and I encour- (Car and Parrinello, 1985) was formulated, there were age readers to look beyond the horizons of their particular just 77 publications! In this article, I revisit the period interest. Finally, I note that both Burke (Burke, 2012) before 1990 and focus on aspects of the development since and Becke (Becke, 2014) are uneasy about some recent then. Recent perspectives on density functional theory developments, and I shall raise my own questions about have been given by Kieron Burke (Burke, 2012) and Axel the future. Becke (Becke, 2014), and much more detailed information is available in monographs and review articles cited there. A recent issue of the Journal of Chemical Physics cele- II. THE DENSITY AS BASIC VARIABLE brates fifty years of density functional theory, and the ar- ticles range across many topics of current interest (Yang, The recent books by Gino Segr`e(Segr`e, 2007) and Gra- 2014). A recent review of solid state applications of DF ham Farmelo (Farmelo, 2009) give fascinating accounts theory is provided by (Hasnip et al., 2014). I have used of the development of quantum mechanics in the years numerous citations of participants in (and observers of) following 1926. Methods for finding approximate solu- this story to illustrate how perspectives of the approach tions of the Schr¨odinger equation followed soon after the have changed over time. The two applications I discuss equations were published and have had a profound effect are DF simulations of systems that were unimaginable on chemistry and condensed matter physics ever since. only a few years ago. A method for calculating the wave function of an atom was developed by Hartree (Hartree, 1928a,b). The “Hartree approximation” to the many-electron wave function is a product of single-particle functions, 1 A similar plot is given by Burke (Burke, 2012) for two popular approximations used in DF calculations.