Relativistic unitary coupled cluster method for ground-state molecular properties

TL;DR

This paper introduces a relativistic unitary coupled cluster method for accurately calculating ground-state properties of heavy-element molecules, improving agreement with experimental data over traditional methods.

Contribution

It develops a non-perturbative commutator-based UCC approach that outperforms perturbative methods and conventional CC in predicting molecular properties.

Findings

qUCCSD shows better agreement with experiments than UCC3.

Non-perturbative qUCC captures relaxation effects more effectively.

Method improves accuracy of molecular property calculations for heavy elements.

Abstract

We propose a relativistic unitary coupled cluster (UCC) expectation value approach for computing first-order properties of heavy-element systems. Both perturbative (UCC3) and non-perturbative (qUCC) commutator-based formulations are applied to evaluate ground-state properties, including the permanent dipole moment (PDM), magnetic hyperfine structure (HFS) constant, and electric field gradient (EFG). The results are compared with available experimental data and those from conventional coupled cluster (CC) calculations. The non-perturbative commutator-based approach truncated at the singles and doubles level (qUCCSD) exhibits markedly better agreement with both CCSD and experiment than the perturbative UCC3 method, likely due to its improved treatment of relaxation effects.