Time dependent second order Green's function theory for neutral excitations

TL;DR

This paper introduces a time dependent second order Green's function theory (GF2) for calculating neutral excitations in molecules, offering improved accuracy over traditional methods especially for charge transfer states.

Contribution

The authors develop a novel GF2-based approach and derive a Bethe-Salpeter-like equation for better excited state calculations, demonstrating its advantages over existing methods.

Findings

GF2-BSE outperforms CIS and TDHF for charge transfer excitations

GF2-BSE is comparable to CIS(D) in accuracy

Method shows promise for accurate molecular excitation calculations

Abstract

We develop a time dependent second order Green's function theory (GF2) for calculating neutral excited states in molecules. The equation of motion for the lesser Green's function (GF) is derived within the adiabatic approximation to the Kadanoff-Baym (KB) equation using the second order Born approximation for the self-energy. In the linear response regime, we recast the time dependent KB equation into a Bethe-Salpeter-like equation (GF2-BSE), with a kernel approximated by the second order Coulomb self-energy. We then apply our GF2-BSE to a set of molecules and atoms and find that GF2-BSE is superior to configuration interaction with singles (CIS) and/or time dependent Hartree-Fock (TDHF), particularly for charge transfer excitations, and is comparable to CIS with perturbative doubles (CIS(D)) in most cases.