Spin-orbit coupling and electron correlation in relativistic configuration interaction and coupled-cluster methods

► Performances of Kramers restricted (KR) and spin-orbit (SO), CI and CC are compared. ► KR-CI and CC exhibit good qualities with respect to increase in spin-orbit coupling. ► SOCI requires triple or quadruple excitations to generate reasonable results. ► SOCC shows a negligible dependence on the strength of spin-orbit coupling. ► Success of SOCC is due to the effective recovery of the spin-orbit energy by SOCCS.