An Efficient Fock Space Multi-reference Coupled Cluster Method based on Natural Orbitals: Theory, Implementation, and Benchmark

TL;DR

This paper introduces a natural orbital-based implementation of the Fock space coupled-cluster method that reduces computational costs and improves performance in excited state calculations, especially for large systems.

Contribution

The authors develop a new natural orbital-based Fock space coupled-cluster method that retains key properties and offers computational advantages over existing methods.

Findings

Significant reduction in computational cost.

Excellent performance for valence, Rydberg, and charge-transfer states.

Superior to equation of motion coupled cluster for large excited states.

Abstract

We present a natural orbital-based implementation of the intermediate Hamiltonian Fock space coupled-cluster method for (1,1) sector of Fock space. The use of natural orbital significantly reduces the computational cost and can automatically choose an appropriate active space. The new method retains the charge transfer separability of the original intermediate Hamiltonian Fock space coupled-cluster method and gives excellent performance for valence, Rydberg, and charge-transfer excited states. It offers significant computational advantages over the popular equation of motion coupled cluster method for excitation energy calculations, especially when one is interested in the calculation of large no excited states.