A study of non-iterative triples contributions in relativistic equation-of-motion coupled-cluster calculations using an exact two-component Hamiltonian with atomic mean-field spin-orbit integrals: Application to uranyl and other heavy-element compounds.

Abstract

The implementation of an equation-of-motion coupled-cluster singles and doubles augmented with a noniterative triples correction [EOM-CCSD(T)(a)*] method [D. A. Matthews and J. F. Stanton, J. Chem. Phys. 145, 124102 (2016)] with an exact two-component Hamiltonian using atomic mean-field spin-orbit integrals (X2CAMF) is reported. Benchmark calculations show that both the correction from triple excitations in the cluster operator to the similarity-transformed Hamiltonian and the contribution from triple excitations in excited-state eigenvalue equations make important contributions to computed excitation energies of molecules containing heavy elements. X2CAMF-EOM-CCSD(T)(a)* excitation energies and excited-state properties for low-lying excited states of As2, Sb2, and Bi2 are in significantly better agreement with corresponding experimental values than EOM-CCSD ones. X2CAMF-EOM-CCSD(T)(a)* calculations have also been demonstrated to provide accurate results for excitation energies of low-lying excited states of bare uranyl ion UO2 2+, hereby resolving a long-standing debate on this topic.