Accurate optical properties from first principles: a Quasiparticle Self consistent GW plus Bethe-Salpeter Equation approach

TL;DR

This paper introduces a combined quasiparticle self-consistent GW and Bethe-Salpeter equation method to accurately compute optical spectra, especially for systems poorly described by standard approaches, showing good agreement with experiments.

Contribution

The paper develops a novel all-electron implementation of a self-consistent GW plus Bethe-Salpeter approach for optical properties, improving accuracy for complex materials.

Findings

Accurately reproduces optical spectra of NiO and Ge.

Performance comparable to standard GW+BSE for well-described systems.

Identifies vertex corrections and electron-phonon interactions as sources of bandgap overestimation.

Abstract

We present an approach to calculate the optical absorption spectra that combines the quasiparticle self-consistent GW method [Phys. Rev. B, 76 165106 (2007)] for the electronic structure with the solution of the ladder approximation to the Bethe-Salpeter equation for the macroscopic dielectric function. The solution of the Bethe-Salpeter equation has been implemented within an all-electron framework, using a linear muffin-tin orbital basis set, with the contribution from the non-local self-energy to the transition dipole moments (in the optical limit) evaluated explicitly. This approach addresses those systems whose electronic structure is poorly described within the standard perturbative GW approaches with as a starting point density-functional theory calculations. The merits of this approach have been exemplified by calculating optical absorption spectra of a strongly correlated transition metal oxide, NiO, and a narrow gap semiconductor, Ge. In both cases, the calculated spectrum is in good agreement with the experiment. It is also shown that for systems whose electronic structure is well-described within the standard perturbative GW, such as Si, LiF and h-BN, the performance of the present approach is in general comparable to the standard GW plus Bethe-Salpeter equation. It is argued that both vertex corrections to the electronic screening and the electron-phonon interaction are responsible for the observed systematic overestimation of the fundamental bandgap and spectrum onset.