The SOC constants between the lowest triplet ( 3 P ) and lowest singlet states ( 1 D) of C, Si, Ge and Sn obtained by SOC-MRSF are presented in Fig. . The corresponding experimental SOC couplings of C and Si are 13.98 and 73.70 cm -1 . The calculated SOC constants are generally insensitive to the choice of XC functionals, while it is gradually affected by the size of basis sets.