Unitary Coupled-Cluster theory for the treatment of molecules in strong magnetic fields

TL;DR

This paper introduces a unitary Coupled-Cluster approach to address unphysical complex energies in molecules subjected to strong magnetic fields, ensuring real-valued energies and improving upon traditional CC methods.

Contribution

The authors develop and implement finite-field UCC2 and UCC3 methods, demonstrating their effectiveness compared to conventional CC techniques in challenging molecular scenarios.

Findings

UCC methods produce real energies where traditional CC fails.

UCC2 and UCC3 show improved accuracy in strong magnetic fields.

Application to molecules like methylidyne ion, water, and boric acid validates the approach.

Abstract

In Coupled-Cluster (CC) theory, unphysical complex energies may arise in the presence of strong magnetic fields, near conical intersections, or in systems exhibiting complex Abelian point group symmetries. This issue originates from the non-Hermitian nature of the CC energy expression. A promising solution is provided by unitary Coupled-Cluster (UCC) theory, which retains the advantages of an exponential parameterization while ensuring real-valued energy eigenvalues. In this work, we present an implementation of finite-field second-order (ff-UCC2) and third-order (ff-UCC3) UCC theory. We assess the performance of these truncation levels in comparison to conventional finite-field CC methods, using the methylidyne ion, water, and boric acid.