3 P 1 state can be a good measure of relativistic effects. The absolute errors in the gaps as compared to experimental data as well as previous high-level fully relativistic four-component (4c-) CASSCF and 4c-CASPT2 calculations are presented in Fig. . The experimental 3 P 1 -3 P 0 splittings are 16.4 cm -1 (C), 77.1 cm -1 (Si), 557.1 cm -1 (Ge), and 1691.8 cm -1 (Sn). The three different exchange-correlation (XC) functionals of PBE0, BH&HLYP and M06-2X were adopted in combination with various basis sets. For all calculations, the Douglas-Kroll (DK) second-order transformation was utilized for the scalar relativistic effect. In the case of C, the error decreases with larger basis sets, showing the importance of basis set. On the other hand, the XC functional dependencies are relatively smaller. Interestingly, the combination of BH&HLYP/cc-pVTZ yields a value with near zero error. In the case of Si, while the errors of both PBE0 and BH&HLYP functionals are generally small and insensitive to the basis sets, those of M06-2X show quite strong basis set dependencies. As in the case of C, the BH&HLYP/cc-pVTZ combination gives the smallest error. For the heavier element of Ge, BH&HLYP functional performs well and gives errors similar to those of 4c-CASSCF and 4c-CASPT2. It is interesting to see that the particular combination of BH&HLYP/cc-pVTZ consistently yielded a near-perfect prediction even in Ge. The better performance of BH&HLYP than the other functionals can be attributed to the formulation of MRSF. As was pointed out by Huix-Rotllant et al., within the widely used collinear (one-component) SF formalism, the configurations obtained by different SF transitions couple through the exact exchange only. As the current implementation of MRSF utilizes the same collinear formalism, it requires a larger fraction of the exact exchange, such as in the BHHLYP functional.