Formalism

Orbital-dependent functionals

Optimally-tuned range-separated hybrid functionals

Local hybrid functionals

Ensemble-generalized functionals

Simulated photoelectron spectroscopy

Dispersion corrections

Simulated doping

 

Orbital-dependent functionals
 

First principles electronic structure calculations, based only on the periodic table and the laws of quantum mechanics, have made large strides in recent decades and have become the foundation for the understanding of a huge variety of physical and chemical systems.

Much of this progress has been due to density functional theory (DFT), which has emerged as the work-horse approach for real-world materials (as opposed to model systems). DFT is an approach to the many-electron problem in which the electron density, rather than the many-electron wave function, plays a central role. It has become the method of choice for electronic structure calculations across an unusually wide variety of fields, from organic chemistry to condensed matter physics. There are two main reasons for the spectacular success of DFT:
 

  • First and foremost, DFT offers the only currently known practical way for first principles calculations of systems with many thousands of electrons.
     
  • Second, it enhances our understanding by relying on relatively simple, accessible quantities that are easily visualized even for very large systems.
     

DFT has progressed from a formal approach to a practical one by virtue of the Kohn-Sham equations. These constitute a mapping of the original N-electron Schrödinger equation into an effective set of N one-electron Schrödinger-like equations, where all non-classical electron interactions (i.e., exchange and correlation) are subsumed into an additive one-electron potential, known as the exchange-correlation potential. The latter is the functional derivative of the exchange-correlation energy, which is a functional of the density. This mapping is exact in principle, but always approximate in practice. Progress therefore hinges critically on our ability to obtain more accurate approximations for exchange-correlation functionals that are applicable across a wide range of systems.

We believe that research into orbital-dependent density functionals is one of the most promising arenas in modern density functional theory. In such functionals, the exchange-correlation energy is expressed explicitly in terms of Kohn-Sham orbitals and is only an implicit functional of the density. This allows maximal freedom in functional construction and offers a real hope for alleviating some of the most serious difficulties associated with present day treatments of exchange and correlation within DFT. Furthermore, orbital-dependent functionals can be employed fully within the original Kohn-Sham framework, in which case the exchange-correlation potential is derived using the optimized effective potential equation. However, they can also be employed using the generalized Kohn-Sham framework, in which case one obtains a non-local potential that corresponds to mapping the original many-electron problem into one of partially interacting electrons. A leading example of those, although not always recognized as such, is the so-called hybrid functionals, where a fraction of exact exchange is “mixed in” with a fraction of explicitly density-dependent exchange. While the Kohn-Sham mapping is unique, there are many generalized Kohn-Sham maps. This additional flexibility allows one to choose the best mapping for a given task.

Our group is actively engaged in constructing, testing, benchmarking, and applying to complex systems several important classes of orbital-dependent functionals.

  • For a comprehensive review article on the topic, see:

Optimally-tuned range-separated hybrid functionals

 

In recent years we have been developing and employing functionals based on the concept of optimal-tuning of a range-separated hybrid functional. In this approach, one separates the electron-repulsion into short- and long-range components, treating the short-range so as to achieve a good balance between exchange and correlation, using semi-local approximations (possibly with short-range exact-exchange), but emphasizing exact-exchange in the long-range so as to obtain the correct asymptotic potential. Optimal-tuning means that the range-separation parameter (roughly, the cross-over point from short to long range) is an adjustable, system-dependent parameter (rather than a universal one). This parameter is obtained non-empirically based on the satisfaction of physical constraints, typically the ionization potential theorem and related properties. This allowed us to solve several related problems that plagued density functional theory, including the infamous gap problem (for finite systems) and the charge-transfer excitation problem.

Recent highlights of this line of research include:

  • Solving the charge-transfer excitation problem, in all its forms:
    • Full charge transfer:
    • Partial charge transfer:
      • T. Stein, L. Kronik, and R. Baer, "Prediction of charge-transfer excitations in coumarin-based dyes using a range-separated functional tuned from first principles", J. Chem. Phys. 131, 244119 (2009).
    • Charge-transfer like scenarios:
    • Solving the gap problem for finite systems:
      • S. Refaely-Abramson, R. Baer and L. Kronik, "Fundamental and excitation gaps in molecules of relevance for organic photovoltaics from an optimally tuned range-separated hybrid functional”, Phys. Rev. B 84, 075144 (2011). Selected as “Editor’s suggestion”.
      • T. Stein, H. Eisenberg, L. Kronik, and R. Baer, "Fundamental gaps of finite systems from the eigenvalues of a generalized Kohn-Sham method", Phys. Rev. Lett.,105, 266802 (2010).
    • Generalization to molecular solids
      • S. Refaely-Abramson, M. Jain, S. Sharifzadeh, J. B. Neaton, L. Kronik, “Solid-state excitonic effects predicted from optimally-tuned time-dependent range-separated hybrid density functional theory”, Phys. Rev. B (Rapid Comm.) 92, 081204(R) (2015).
      • D. Lüftner, S. Refaely-Abramson, M. Pachler, R. Resel, M. G. Ramsey, L. Kronik, and P. Puschnig, “Experimental and theoretical electronic structure of quinacridone”, Phys. Rev. B 90, 075204 (2014).
      • S. Refaely-Abramson, S. Sharifzadeh, M. Jain, R. Baer, J. B. Neaton, L. Kronik, “Gap renormalization of molecular crystals from density functional theory”, Phys. Rev. B (Rapid Comm.), 88, 081204 (2013).
    • Generalization and application to outer-valence electronic structure
    • Applications to amino acids and peptide structures
      • (invited paper) S. Sarkar and L. Kronik, “Ionization and (de-)protonation energies of gas-phase amino acids from an optimally tuned range-separated hybrid functional”, Mol. Phys. (Special Issue in Honor of Professor Andreas Savin), in press (2016).
      • M. Eckshtain-Levi, E. Capua, S. Refaely-Abramson, S. Sarkar, Y. Gavrilov, S. Mathew, Y. Paltiel, Y. Levy, L. Kronik, and R. Naaman, “Cold Denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers”, Nature Comm. 7, 10744 (2016).
      • L. Sepunaru, S. Refaely-Abramson, R. Lovrinčić, Y. Gavrilov, P. Agrawal, Y. Levy, L. Kronik, I. Pecht, M. Sheves, and D. Cahen, “Electronic Transport via Homo-peptides: The Role of Side Chains and Secondary Structure”, J. Am. Chem. Soc. 137, 9617 (2015). Highlighted in Phys.org.
    • And finally some caveats
      • A. Karolewski, L. Kronik, and S. Kümmel “Using optimally-tuned range separated hybrid functionals in ground-state calculations: consequences and caveats”, J. Chem. Phys. 138, 204115 (2013).

Local hybrid functionals
 

A different area of much interest to us is the development of novel local hybrid functionals, where the fraction of exact exchange is spatially-dependent, following variations in the density and orbitals. In particular, we are interested in using this to develop non-local correlation functionals compatible with exact exchange, that are free of one-electron self-interaction, respect constraints derived from uniform coordinate scaling, and exhibit the correct asymptotic behavior of the exchange-correlation energy.

Ensemble-generalized functionals

 

We have been heavily involved in the study of the derivative discontinuity – a quirky property of the exchange-correlation potential, which makes it “jump” by a constant across an integer number of electrons. Traditionally, simple approximations to the exchange-correlation functional were thought to be devoid of this derivative discontinuity, with severe implications to the computation of bandgaps from eigenvalues, where this must be taken into account. Recently, we have discovered that in fact with a simple ensemble-generalization all functionals possess a derivative discontinuity and that this can be used to extract relatively accurate molecular gaps even from very simple functionals.

Recent highlights of this line of research include:

  • Demonstrating the existence and importance of the derivative discontinuity in the potential of a dissociating molecule:
    • A. Makmal, S. Kümmel, and L. Kronik, "Dissociation of diatomic molecules and the exact-exchange Kohn-Sham potential: the case of LiF", Phys. Rev. A 83, 062512 (2011).
  • Revealing the derivative discontinuity from ensemble considerations, including implications for orbital energies and gaps:
    • E. Kraisler and L. Kronik, “Fundamental gaps with approximate density functionals: the derivative discontinuity revealed from ensemble considerations”, J. Chem. Phys. 140, 18A540 (2014).
    • T. Schmidt, E. Kraisler, L. Kronik, S. Kümmel, “One-electron self-interaction and the asymptotics of the Kohn-Sham potential: an impaired relation” Phys. Chem. Chem. Phys. 16, 14357 (2014).
    • E. Kraisler and L. Kronik, “Piecewise linearity of approximate density functionals revisited: Implications for frontier orbital energies”, Phys. Rev. Lett. 110, 126403 (2013).
    • E. Kraisler, T. Schmidt, S. Kümmel, and L. Kronik, “Effect of ensemble generalization on the highest-occupied Kohn-Sham eigenvalue”, J. Chem. Phys. 143, 104105 (2015).

Simulated photoelectron spectroscopy

 

We have been exploring the pros and cons of using various functionals, including explicitly density-dependent ones, and different conventional and novel hybrid ones, as a tool for quantitative simulations of photoelectron spectroscopy.

Recent highlights of this line of research include:

  • For an overview article, see:
    • L. Kronik and S. Kümmel, "Gas-phase valence-electron photoemission spectroscopy using density functional theory”, in Topics of Current Chemistry: First Principles Approaches to Spectroscopic Properties of Complex Materials, C. di Valentin, S. Botti, M. Coccoccioni, Editors (Springer, Berlin, 2014).
  • For investigations of phthaolcyanines, see:
    • N. Marom, O. Hod, G. E. Scuseria, and L. Kronik, "Electronic Structure of Copper Phthalocyanine: a Comparative Density Functional Theory Study", J. Chem. Phys. 128, 164107 (2008).
    • (invited paper) N. Marom and L. Kronik, "Density Functional Theory of Transition Metal Phthalocyanines. I: Electronic Structure of NiPc and CoPc- Self-Interaction Effects", Appl. Phys. A 95, 159 (2009). (Special Issue on Organic Materials for Electronic Applications).
    • (invited paper) N. Marom and L. Kronik, "Density Functional Theory of Transition Metal Phthalocyanines. II: Electronic Structure of MnPc and FePc - Symmetry and Symmetry Breaking", Appl. Phys. A 95, 165 (2009). (Special Issue on Organic Materials for Electronic Applications)
    • D. A. Egger, S. Weismann, S. Refaely-Abramson, S. Sharifzadeh, M. Dauth, R. Baer, S. Kümmel, J. B. Neaton, E. Zojer, L. Kronik, “Outer-valence electron spectra of prototypical aromatic heterocycles from an optimally-tuned range-separated hybrid functional”, J. Chem. Theo. Comp. 10, 1934 (2014).
  • For investigation of aromatic molecules and their derivatives, see:
    • D. Lüftner, S. Refaely-Abramson, M. Pachler, R. Resel, M. G. Ramsey, L. Kronik, and P. Puschnig, “Experimental and theoretical electronic structure of quinacridone”, Phys. Rev. B 90, 075204 (2014).
    • S. Refaely-Abramson, S. Sharifzadeh, N, Govind, J. Autschbach, J. B. Neaton, R. Baer and L. Kronik, “Quasiparticle spectra from a non-empirical optimally-tuned range-separated hybrid density functional”, Phys. Rev. Lett. 109, 226405 (2012).
    • T. Körzdörfer, S. Kümmel, N. Marom, and L. Kronik, "When to trust photoelectron spectra from Kohn-Sham eigenvalues: the case of organic semiconductors", Phys. Rev. B (Rapid Comm.) 79, 201205 (2009).
    • N. Dori, M. Menon, L. Kilian, M. Sokolowski, L. Kronik, and E. Umbach, "Valence Electronic Structure of Gas Phase 3,4,9,10-perylene tetracarboxylic-acid-dianhydride (PTCDA): Experiment and Theory", Phys. Rev. B 73, 195208 (2006).

Dispersion corrections

 

Last but not at all least, we are interested in further developments and applications of pair-wise and beyond-pair-wise dispersion corrections both standard and optimally-tuned range-separated hybrid functionals, as a means of incorporating long-range correlation that is essential to the capture of weak interactions.
Recent highlights of this line of research include:

  • For an overview article, see:
    • L. Kronik and A. Tkatchenko, “Understanding molecular crystals with dispersion-inclusive density-functional theory: pair-wise corrections and beyond”, Acct. Chem. Research (Special Issue on DFT Elucidation of Materials Properties), Acc. Chem. Res, in press. 
  • For a combination of this approach with optimally-tuned range-separated hybrid functionals, see:
    • P. Agrawal, A. Tkatchenko, and L. Kronik, “Pair-wise and many-body dispersive interactions coupled to an optimally-tuned range-separated hybrid functional", J. Chem. Theo. Comp., 9, 3473 (2013).

Simulated doping

 

The inclusion of the global effects of semiconductor doping poses a unique challenge for first-principles simulations, because the typically low concentration of dopants renders an explicit treatment intractable. Furthermore, the width of the space-charge region (SCR) at charged surfaces often exceeds realistic supercell dimensions. Recently, we developed a multiscale technique that fully addresses these difficulties. It is based on the introduction of a charged sheet, mimicking the SCR-related field, along with free charge which mimics the bulk charge reservoir, such that the system is neutral overall. These augment a slab comprising “pseudoatoms” possessing a fractional nuclear charge matching the bulk doping concentration. Self-consistency is reached by imposing charge conservation and Fermi level equilibration between the bulk, treated semiclassically, and the electronic states of the slab, which are treated quantum-mechanically. The method, called CREST—the charge-reservoir electrostatic sheet technique—can be used with standard electronic structure codes.

  • For recent highlights of this approach see:
    • O. Sinai, O. T. Hofmann, P. Rinke, M. Scheffler, G. Heimel, and L. Kronik, “Multi-scale approach to the electronic structure of doped semiconductor surfaces”, Phys. Rev. B 91, 075311 (2015). Selected as “Editor’s suggestion”.
    • O. Sinai and L. Kronik, ”Simulated doping of Si from first principles using pseudo-atoms", Phys. Rev. B 87, 235305 (2013).