An Excited-State-Specific Pseudoprojected Coupled-Cluster Theory

TL;DR

This paper introduces an excited-state-specific coupled-cluster method that optimizes orbitals and amplitudes for individual excited states, improving accuracy over existing perturbation theories.

Contribution

It develops a pseudoprojected coupled-cluster approach tailored for excited states, combining orbital and amplitude optimization with efficient computational scaling.

Findings

Method improves over excited-state second order perturbation theory.

Incorporates $N^5$-cost perturbative corrections for better accuracy.

Maintains size extensivity and $N^6$ scaling similar to ground state CCSD.

Abstract

We present an excited-state-specific coupled-cluster approach in which both the molecular orbitals and cluster amplitudes are optimized for an individual excited state. The theory is formulated via a pseudoprojection of the traditional coupled-cluster wavefunction that allows correlation effects to be introduced atop an excited state mean field starting point. The approach shares much in common with ground state CCSD, including size extensivity and an $N^6$ cost scaling. Preliminary numerical tests show that, when augmented with $N^5$-cost perturbative corrections for key terms, the method can improve over excited-state-specific second order perturbation theory in valence, charge transfer, and Rydberg states.