Excitation Energies of UO22+, NUO+, and NUN Based on Equation-of-Motion Coupled-Cluster Theory with Spin- Orbit Coupling

Obtaining reliable excitation energies of the isoelectronic series, UO22+, NUO+, and UN2, is a challenge in quantum chemistry calculations due to importance of electron correlation and spin-orbit coupling (SOC). Vertical spin-free and spin-orbit coupled excitation energies of these molecules are calculated in this work using equation-of-motion coupled-cluster approach at the CC singles and doubles level (EOM-CCSD). SOC is included in post-SCF calculations and this treatment has been shown to provide SOC effects with high accuracy for heavy elements. Excitation energies for UO22+ with the present approach are in good agreement with previous results using linear response CCSD, multiconfiguration perturbation theory (CASPT2), and multireference configuration interaction (MRCI). As for NUO+ and UN2, excitation energies with CASPT2 from two previous calculations differ significantly and the present results usually lie between these two sets of CASPT2 results. On the other hand, excitation energies with intermediate Hamiltonian Fock space CC method (IHFSCC) are generally too small compared with our results. This work provides new estimates on excitation energies of these molecules and it could be helpful in investigating spectroscopic and luminescence properties of larger uranium complexes.