The results in Table show anticipated trends: the magnitude of SOCC increases for heavy elements. Moreover, for these systems the contribution of the two-electron part is smaller than for typical organic molecules. The magnitude of SOCCs between the degenerate pairs of states ( 2 Π or 2 E) is slightly larger than between π-and σ manifolds, which probably can be rationalized by considering the shapes of the respective MOs and El-Sayed's rule .