As a representative third-row system, 4-thiothymine is adopted for SOC-MRSF to validate its SOC predictions. The lowest singlet excited state (S 1 ) has a 1 nπ * character with the sulfur lone pair orbitals, which is nearly degenerate with the two lowest triplet 3 nπ * and 3 ππ * states at the S 1 minimum geometry. Depending on the exchange-correlation functionals for TDDFT and CASPT2 methods, it was reported that the 1 nπ * can be either S 1 or S 2 . Although they are nearly degenerate, the 3 ππ * and 3 nπ * are T 1 and T 2 , respectively, showing that the corresponding states are sensitive to the choice of theories. To explore functional dependencies, the B3LYP, PBE0, BH&HLYP and M06-2X functionals were utilized in the current SOC-MRSF calculations. Furthermore, in order to study the geometric As reference values, the single point SOC calculations by the SO-GMC-QDPT2 /cc-pVTZ level were also performed at the S 1 minimum geometry optimized by the MRSF/6-31G* with four different functionals. The optimized S 1 geometries of 4-thiothymine by B3LYP, PBE0, BH&HLYP and M06-2X functionals are shown in Figure . While the optimized geometries of B3LYP, PBE0