Spin-flip Bethe-Salpeter equation approach for ground and excited states of open-shell molecules and defects in solids

TL;DR

This paper introduces a spin-flip Bethe-Salpeter equation method for accurately calculating ground and excited states of open-shell molecules and defects in solids, applicable in periodic systems without embedding.

Contribution

It presents a novel spin-flip approach within the Bethe-Salpeter framework that handles open-shell systems in periodic boundary conditions without subspace selection.

Findings

Accurately models torsion potential-energy surface of ethylene.

Predicts optical excitations of diamond NV$^{-}$ center with high agreement.

Shows low to moderate spin contamination in results.

Abstract

Open-shell systems such as magnetic molecules or defects with a triplet ground state are challenging to describe in electronic structure methods, but are of great interest for quantum information applications. We demonstrate a spin-flip approach within the Bethe-Salpeter equation to calculate ground and excited states of open-shell molecules and defected solids. The approach works in periodic boundary conditions without any need for embedding or selection of a subspace. Our benchmark results for the torsion potential-energy surface of ethylene and the optical excitations of the diamond NV$^{-}$ center show excellent agreement with the literature, and a low or moderate level of spin contamination.