# SternheimerGW: a program for calculating GW quasiparticle band structures and spectral functions without unoccupied states

## TL;DR

SternheimerGW is a computational tool that calculates GW quasiparticle band structures and spectral functions without requiring unoccupied states, using a linear response approach for improved accuracy and efficiency.

## Contribution

It introduces a method based on Sternheimer equations to compute GW properties without summing over unoccupied states, enhancing computational efficiency and applicability.

## Key findings

- Accurate spectral functions for solids achieved without unoccupied states.
- Efficient parallel implementation integrated with Quantum Espresso.
- Applicable to indirect band gap semiconductors and angle-resolved spectra.

## Abstract

The SternheimerGW software uses time-dependent density-functional perturbation theory to evaluate GW quasiparticle band structures and spectral functions for solids. Both the Green's function G and the screened Coulomb interaction W are obtained by solving linear Sternheimer equations, thus overcoming the need for a summation over unoccupied states. The code targets the calculation of accurate spectral properties by convoluting G and W using a full frequency integration. The linear response approach allows users to evaluate the spectral function at arbitrary electron wavevectors, which is particularly useful for indirect band gap semiconductors and for simulations of angle-resolved photoelectron spectra. The software is parallelized efficiently, integrates with version 6.3 of Quantum Espresso, and is continuously monitored for stability using a test farm.