Accurate Prediction of Band Structure of FeS2: A Hard Quest of Advanced First-Principles Approaches

The pyrite and marcasite polymorphs of FeS2 have attracted considerable interests for their potential applications in optoelectronic devices because of their appropriate electronic and optical properties. Controversies regarding their fundamental band gaps remain in both experimental and theoretical materials research of FeS2. In this work, we present a systematic theoretical investigation into the electronic band structures of the two polymorphs by using many-body perturbation theory with the GW approximation implemented in the full-potential linearized augmented plane waves (FP-LAPW) framework. By comparing the quasi-particle (QP) band structures computed with the conventional LAPW basis and the one extended by high-energy local orbitals (HLOs), denoted as LAPW + HLOs, we find that one-shot or partially self-consistent GW (G0W0 and GW0, respectively) on top of the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation with a converged LAPW + HLOs basis is able to remedy the artifact reported in the previous GW calculations, and leads to overall good agreement with experiment for the fundamental band gaps of the two polymorphs. Density of states calculated from G0W0@PBE with the converged LAPW + HLOs basis agrees well with the energy distribution curves from photo-electron spectroscopy for pyrite. We have also investigated the performances of several hybrid functionals, which were previously shown to be able to predict band gaps of many insulating systems with accuracy close or comparable to GW. It is shown that the hybrid functionals considered in general fail badly to describe the band structures of FeS2 polymorphs. This work indicates that accurate prediction of electronic band structure of FeS2 poses a stringent test on state-of-the-art first-principles approaches, and the G0W0 method based on semi-local approximation performs well for this difficult system if it is practiced with well-converged numerical accuracy.

Band gaps of crystalline solids from Wannier-localization–based optimal tuning of a screened range-separated hybrid functional

Accurate prediction of fundamental band gaps of crystalline solid-state systems entirely within density functional theory is a long-standing challenge. Here, we present a simple and inexpensive method that achieves this by means of nonempirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron from an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked against experiment for a set of systems ranging from narrow band-gap semiconductors to large band-gap insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 electronvolts (eV), and is found to yield quantitative accuracy across the board, with a mean absolute error of ∼0.1 eV and a maximal error of ∼0.2 eV.

Уширение и сдвиг линий основной колебательно-вращательной полосы молекул фтористого водорода молекулами SF-=SUB=-6-=/SUB=- и H-=SUB=-2-=/SUB=-

Foreign gas broadening and shift of the vibration-rotation lines in the fundamental band of hydrogen fluoride have been investigated. Using mathematical modeling of measuring the absorption of radiation of an electric discharge HF laser, the possibility of correct calculations of both broadening and shift of HF lines by foreign gases were determined. Measurements for two foreign gases (Ar and N2 for which there are literature data) were made in order to test this approach. The obtained results have been well matched with existing literature data. After that, measurements were carried out on two transitions P(7) and P(8) for SF6 (for which neither broadening nor the shift are known) and for H2 (only broadening is known for overtone band). The obtained results were compared with scarce literature data.

Anisotropic optical properties of GeS investigated by optical absorption and photoreflectance

We present a comprehensive study of bulk GeS optical properties and their anisotropy, by investigation of the fundamental band gap of the material and the energetically higher direct transitions.