Non-iterative Triples for Transcorrelated Coupled Cluster Theory

TL;DR

This paper introduces a non-iterative triples correction method for transcorrelated coupled cluster theory, efficiently approximating three-body interactions to improve computational feasibility.

Contribution

It presents a novel approximation for the three-body operator in transcorrelated coupled cluster theory, enabling non-iterative triples corrections with reduced memory and computational costs.

Findings

The approximation yields results comparable to full triples calculations.

The method reduces memory bottlenecks and runtime scaling.

Comparison with quadruple excitation calculations validates the approach.

Abstract

We present an implementation of a perturbative triples correction for the coupled cluster ansatz including single and double excitations based on the transcorrelated Hamiltonian. Transcorrelation introduces explicit electron correlation in the electronic Hamiltonian through similarity transformation with a correlation factor. Due to this transformation, the transcorrelated Hamiltonian includes up to three-body couplings and becomes non-Hermitian. Since the conventional coupled cluster equations are solved by projection, it is well suited to harbor non-Hermitian Hamiltonians. The arising three-body operator, however, creates a huge memory bottleneck and increases the runtime scaling of the coupled cluster equations. As it has been shown that the three-body operator can be approximated, by expressing the Hamiltonian in the normal-ordered form, we investigate this approximation for the perturbative triples correction. Results are compared with a code-generation based transcorrelated coupled cluster implementation up to quadruple excitations.