A Similarity Transformed Second-order Approximate Coupled Cluster Method for the Excited States: Theory, Implementation, and Benchmark

TL;DR

This paper introduces a novel similarity-transformed second-order coupled cluster method that improves excitation energy calculations, especially for Rydberg and charge-transfer states, while maintaining computational efficiency.

Contribution

The paper develops a new similarity-transformed CC2 method that better captures higher-order effects and improves accuracy for excited states, with an MBPT2 variant also proposed.

Findings

Significant improvement in Rydberg and charge-transfer excitation energies.

Retains good performance for valence excited states.

Maintains charge-transfer separability advantages.

Abstract

We present a novel and cost-effective approach of using a second similarity transformation of the Hamiltonian to include the missing higher-order terms in the second-order approximate coupled cluster singles and doubles (CC2) model. The performance of the newly developed ST-CC2 model has been investigated for the calculation of excitation energies of valence, Rydberg, and charge-transfer excited states. The method shows significant improvement in the excitation energies of Rydberg and charge-transfer excited states as compared to the conventional CC2 method while retaining the good performance of the latter for the valence excited state. The method retains the charge-transfer separability of the CT excited states, which is a significant advantage over the traditional CC2 method. An MBPT2 variant of the new method is also proposed.