Improved Dielectric Response of Solids: Combining the Bethe-Salpeter Equation with the Random Phase Approximation

Amalie H. Søndersted*, Mikael Kuisma, Jakob K. Svaneborg, Mark Kamper Svendsen, Kristian S. Thygesen

*Corresponding author for this work

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Abstract

The Bethe-Salpeter equation (BSE) can provide an accurate description of low-energy optical spectra of insulating crystals - even when excitonic effects are important. However, due to high computational costs it is only possible to include a few bands in the BSE Hamiltonian. As a consequence, the dielectric screening given by the real part of the dielectric function can be significantly underestimated by the BSE. Here, we show that universally accurate optical response functions can be obtained by combining a four-point BSE-like equation for the irreducible polarizability with a two-point Dyson equation that includes the higher-lying transitions within the random phase approximation. The new method is referred to as BSE+. It has a computational cost comparable to the BSE but a much faster convergence with respect to the size of the electron-hole basis. We use the method to calculate refractive indices and electron energy loss spectra for a test set of semiconductors and insulators. In all cases the BSE+ yields excellent agreement with experimental data across a wide frequency range and outperforms both the BSE and the random phase approximation.

Original languageEnglish
Article number026403
JournalPhysical Review Letters
Volume133
Issue number2
Number of pages7
ISSN0031-9007
DOIs
Publication statusPublished - 2024

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