Improving Aufbau Suppressed Coupled Cluster Through Perturbative Analysis

TL;DR

This paper enhances the accuracy of Aufbau suppressed coupled cluster theory for various excitations using perturbative analysis and spin-adaptation, achieving superior results especially for charge transfer states with minimal additional computational cost.

Contribution

It introduces a perturbative approach and spin-adaptation techniques to improve coupled cluster accuracy for excited states without increasing computational complexity.

Findings

Achieves high accuracy in single excitations and charge transfer states.

Outperforms equation-of-motion coupled cluster theory by 0.25 eV for charge transfer.

Partial linearization increases accuracy by reducing unwanted effects.

Abstract

Guided by perturbative analysis, we improve the accuracy of Aufbau suppressed coupled cluster theory in simple single excitations, multi-configurational single excitations, and charge transfer excitations while keeping the cost of its leading-order terms precisely in line with ground state coupled cluster. Combining these accuracy improvements with a more efficient implementation based on spin-adaptation, we observe high accuracy in a large test set of single excitations, and, in particular, a mean unsigned error for charge transfer states that outperforms equation-of-motion coupled cluster theory by 0.25 eV. We discuss how these results are achieved via a systematic identification of which amplitudes to prioritize for single- and multi-configurational excited states, and how this prioritization differs in important ways from the ground state theory. In particular, our data show that a partial linearization of the theory increases accuracy by mitigating unwanted side effects of Aufbau suppression.