In contrast, the intermediate Hamiltonian Fock-space CCSD theory (IHFSCCSD) were applied at the 4c and X2C levels, integrating SO effects at the molecular orbital level. In this study, we compare our X2C-RASCI and RASPT2 results with four previous studies, SO-EOM-CCSD, 41 SO-CASPT2, 39 4c-IHFSCCSD, and 4c-SAOP (TDDFT), where the vertical excitation energies were calculated at a U-O bond distance of 1.708 Å. A comparison of these previous studies with the results of the present work is summarized in Table and Figure .