A configuration interaction correction on top of pair coupled cluster doubles

TL;DR

This paper evaluates the accuracy of a combined pair coupled cluster doubles and configuration interaction method for modeling strongly correlated systems, showing significant improvements over traditional approaches.

Contribution

It introduces and benchmarks a pCCD-CI method with various CI models, including Davidson correction, for better accuracy in strongly correlated systems.

Findings

pCCD-CI methods improve spectroscopic constants over CCSD

Davidson correction enhances accuracy of the models

accuracy is between linearized frozen pCCD and frozen pCCD

Abstract

Numerous numerical studies have shown that geminal-based methods are a promising direction to model strongly correlated systems with low computational costs. Several strategies have been introduced to capture the missing dynamical correlation effects, which typically exploit \textit{a posteriori} corrections to account for correlation effects associated with broken-pair states or inter-geminal correlations. In this article, we scrutinize the accuracy of the pair coupled cluster doubles (pCCD) method extended by configuration interaction (CI) theory. Specifically, we benchmark various CI models, including, at most double excitations against selected CC corrections as well as conventional single-reference CC methods. A simple Davidson correction is also tested. The accuracy of the proposed pCCD-CI approaches is assessed for challenging small model systems such as the \ce{N2} and \ce{F2} dimers and various di- and triatomic actinide-containing compounds. In general, the proposed CI methods considerably improve spectroscopic constants compared to the conventional CCSD approach, provided a Davidson correction is included in the theoretical model. At the same time, their accuracy lies between the linearized frozen pCCD and frozen pCCD variants.