Charge transfer excitations from exact and approximate ensemble Kohn-Sham theory

TL;DR

This paper demonstrates that ensemble Kohn-Sham theory can effectively describe charge transfer excitations, with accurate triplet excitation energies and semi-quantitative singlet results, even in strongly correlated dissociation scenarios.

Contribution

It introduces a method to compute charge transfer excitations using ensemble Kohn-Sham theory and assesses its accuracy with an exactly solvable model.

Findings

Triplet excitations are accurately reproduced across dissociation curves.

Singlet excitations show larger deviations but remain semi-quantitative.

Ensemble components can describe charge transfers effectively.

Abstract

By studying the lowest excitations of an exactly solvable one-dimensional molecular model, we show that components of Kohn-Sham ensembles can be used to describe charge transfers. Furthermore, we compute the approximate excitation energies obtained by using thee exact ensemble densities in the recently formulated ensemble Hartree-exchange theory [Gould and Pittalis, Phys. Rev. Lett. 119, 243001 (2017)]. Remarkably, our results show that triplet excitations are accurately reproduced across a dissociation curve in all cases tested, even in systems where ground state energies are poor due to strong static correlations. Singlet excitations exhibit larger deviations from exact results but are still reproduced semi-quantitatively.