Estimating Excitonic Effects in the Absorption Spectra of Solids: Problems and Insight from a Guided Iteration Scheme

TL;DR

This paper addresses the challenge of accurately predicting excitonic effects in solid optical spectra within time-dependent density-functional theory, proposing an iterative scheme that improves exciton binding energy estimates and allows extraction from RPA spectra.

Contribution

It introduces a guided iterative approximation method that enhances exciton binding energy predictions and enables extraction from simpler spectra, advancing computational optical spectroscopy.

Findings

Improved accuracy in exciton binding energy predictions.

Method to extract exciton energies from RPA spectra.

Reevaluation of existing approximations' reliability.

Abstract

A major obstacle for computing optical spectra of solids is the lack of reliable approximations for capturing excitonic effects within time-dependent density-functional theory. We show that the trustful prediction of strongly bound electron-hole pairs within this framework using simple approximations is still a challenge and that available promising results have to be revisited. Deriving a set of analytical formula we analyze and explain the difficulties. We deduce an alternative approximation from an iterative scheme guided by previously available knowledge, significantly improving the description of exciton binding energies. Finally, we show how one can "read" exciton binding energies from spectra determined in the random phase approximation, without any further calculation.