Although discussions regarding the inclusion of IVO and the comparison between (2h, 2e) and (3h, 3e) will be presented later, the most reliable result in this study is considered to be RASPT2 (3h, 3e) with IVO, which yields values relatively close to those of SO-CASPT2 and SO-EOM- CCSD. However, for the two 2g states, the results are also close to those of IHFSCCSD. As shown later in Figure , these two 2g states consist of a greater number of dominant configurations, which may relate with stronger multiconfigurational effects compared to other states. Considering the 2g states as exceptions, the results suggest that for the low-energy excited states targeted in this study, the method of including electron correlation has a greater impact on the calculated results than whether SO coupling is included from the outset or later.