Libxc: a library of exchange and correlation functionals for density functional theory

The central quantity of density functional theory is the so-called exchange-correlation functional. This quantity encompasses all non-trivial many-body effects of the ground-state and has to be approximated in any practical application of the theory. For the past 50 years, hundreds of such approximations have appeared, with many successfully persisting in the electronic structure community and literature. Here, we present a library that contains routines to evaluate many of these functionals (around 180) and their derivatives.Comment: 15 page

All-electron Exact Exchange Treatment of Semiconductors: Effect of Core-valence Interaction on Band-gap and dd-band Position

Exact exchange (EXX) Kohn-Sham calculations within an all-electron full-potential method are performed on a range of semiconductors and insulators (Ge, GaAs, CdS, Si, ZnS, C, BN, Ne, Ar, Kr and Xe). We find that the band-gaps are not as close to experiment as those obtained from previous pseudopotential EXX calculations. Full-potential band-gaps are also not significantly better for $sp$ semiconductors than for insulators, as had been found for pseudopotentials. The locations of $d$-band states, determined using the full-potential EXX method, are in excellent agreement with experiment, irrespective of whether these states are core, semi-core or valence. We conclude that the inclusion of the core-valence interaction is necessary for accurate determination of EXX Kohn-Sham band structures, indicating a possible deficiency in pseudopotential calculations.Comment: 4 pages 2 fig

Critical role of electronic correlations in determining crystal structure of transition metal compounds

The choice that a solid system "makes" when adopting a crystal structure (stable or metastable) is ultimately governed by the interactions between electrons forming chemical bonds. By analyzing 6 prototypical binary transition-metal compounds we demonstrate here that the orbitally-selective strong $d$-electron correlations influence dramatically the behavior of the energy as a function of the spatial arrangements of the atoms. Remarkably, we find that the main qualitative features of this complex behavior can be traced back to simple electrostatics, i.e., to the fact that the strong $d$-electron correlations influence substantially the charge transfer mechanism, which, in turn, controls the electrostatic interactions. This result advances our understanding of the influence of strong correlations on the crystal structure, opens a new avenue for extending structure prediction methodologies to strongly correlated materials, and paves the way for predicting and studying metastability and polymorphism in these systems.Comment: Main text: 8 pages, 4 figures, 1 table; Supplemental material: 2 pages, 1 figure, 2 table

Si3AlP: A new promising material for solar cell absorber

First-principles calculations are performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si2)0.6(AlP)0.4 alloy [T. Watkins et al., J. Am. Chem. Soc. 2011, 133, 16212.] We find that the ordered CC-Si3AlP with a basic unit of one P atom surrounded by three Si atoms and one Al atom is the most stable one within the experimentally observed unit cell.1 Si3AlP has a larger fundamental band gap and a smaller direct band gap than Si, thus it has much higher absorption in the visible light region. The calculated properties of Si3AlP suggest that it is a promising candidate for improving the performance of the existing Si-based solar cells. The understanding on the stability and band structure engineering obtained in this study is general and can be applied for future study of other nonisovalent and lattice-matched semiconductor alloys

Nonlocal correlations in the vicinity of the α\alpha-γ\gamma phase transition in iron within a DMFT plus spin-fermion model approach

We consider nonlocal correlations in iron in the vicinity of the $\alpha$-$\gamma$ phase transition within the spin-rotationally-invariant dynamical mean-field theory (DMFT) approach, combined with the recently proposed spin-fermion model of iron. The obtained nonlocal corrections to DMFT yield a decrease of the Curie temperature of the $\alpha$ phase, leading to an agreement with its experimental value. We show that the corresponding nonlocal corrections to the energy of the $\alpha$ phase are crucially important to obtain the proximity of energies of $\alpha$ and $\gamma$ phases in the vicinity of the iron $\alpha$-$\gamma$ transformation.Comment: 5 pages, 2 figure