Effects of partial triple excitations in atomic coupled cluster calculations

TL;DR

This paper investigates the impact of higher-order excitations in relativistic coupled cluster calculations for atoms and ions, showing that unitary coupled cluster methods improve the accuracy of ionization potential estimates especially for complex electron configurations.

Contribution

It demonstrates that unitary coupled cluster theory more effectively incorporates higher excitation effects than CCSD(T) in atomic calculations.

Findings

CCSD(T) accurately estimates IPs for simple atoms and ions.

CCSD(T) produces unphysical levels for atoms with partially filled shells.

UCC methods better incorporate higher excitation effects.

Abstract

In this article we study the effects of higher body excitations in the relativistic CC calculations for atoms and ions with one valence electron using Fock-space CCSD, CCSD(T) and its unitary variants. The present study demonstrates that CCSD(T) estimates the ionization potentials (IPs) and the valence electron removal energies quite accurately for alkali atoms and singly ionized alkaline earth ions, but yields unphysical energy levels for atoms and/or ions with partially filled sub-shell like C II. We further demonstrate that the higher body excitation effects can be incorporated more effectively through the unitary coupled cluster theory (UCC) compared to the CCSD(T) method.