Capturing Multireference Excited States by Constrained DFT

TL;DR

This paper introduces an extension of constrained DFT that uses ensemble 1-RDMs to efficiently compute low-lying excited states with multireference character, achieving accuracy comparable to TDDFT.

Contribution

The paper presents a novel mean field method extending XCDFT to ensemble 1-RDMs for better multireference excited state calculations.

Findings

Good description of multireference states

Accuracy comparable to semilocal TDDFT

Maintains computational efficiency

Abstract

The computation of excited electronic states with commonly employed (approximate) methods is challenging, typically yielding states of lower quality than the corresponding ground state for a higher computational cost. In this work, we present a mean field method that extends the previously proposed eXcited Constrained DFT (XCDFT) from single Slater determinants to ensemble 1-RDMs for computing low-lying excited states. The method still retains an associated computational complexity comparable to a semilocal DFT calculation while at the same time is capable of approaching states with multireference character. We benchmark the quality of this method on well-established test sets, finding good descriptions of the electronic structure of multireference states and maintaining an overall accuracy for the predicted excitation energies comparable to semilocal TDDFT.