A study of non-iterative triples contributions in relativistic equation-of-motion coupled-cluster calculations using...

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.