with the SO-GMC-QDPT2 references. The SOC values of GS/ 3 ππ * and 1 nπ * / 3 nπ * are negligibly small as it should be. The reference ROHF orbitals by BH&HLYP and PBE0 functionals are shown in Fig. along with the major configurations of GS, S 1 , T 1 and 2 2 states. It is seen that the n S and π orbitals are mainly composed of S nonbonding and π orbitals, which is the main source of large SOC values.