Improving Aufbau Suppressed Coupled Cluster through Perturbative Analysis.

Guided by perturbative analysis, we improve the accuracy of Aufbau suppressed coupled cluster theory in simple single excitations, multiconfigurational 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 multiconfigurational 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.