Intermediate Hamiltonian Fock-space coupled-cluster theory for excitation energies, double ionization potentials, and double electron attachments with spin–orbit coupling

Abstract

The intermediate Hamiltonian Fock-space coupled-cluster methods at the singles and doubles level (IHFSCCSD) for excitation energies (EEs) in the (1p,1h) sector, double ionization potentials (DIPs) in the (0p,2h) sector and double electron attachements (DEAs) in the (2p,0h) sector of the Fock space are implemented based on the CCSD method with spin-orbit coupling (SOC) included in the post-Hartree-Fock treatment using a closed-shell reference in this work. The active space is chosen to contain those orbitals that have the largest contribution to principal ionized or electron-attached states obtained from equation-of-motion coupled-cluster calculations. Both time-reversal symmetry and spatial symmetry are exploited in the implementation. Our results show that accuracy of IHFSCCSD results is closely related to the active space and sufficiency of the active space can be assessed from the percentage of transitions within the active space. In addition, unreasonable results may be encountered when ionized or electron-attached states with a somewhat larger contribution from double excitations are included to determine the active space and cluster operators in the (0p,1h) or (1p,0h) sector of the Fock space. A larger active space may be required to describe SO splitting reliably than that in scalar-relativistic calculations in some cases. The IHFSCCSD method with SOC developed in this work can provide reliable results for heavy-element systems when a sufficient active space built upon principal IP/EA states is adopted.