The existing methods to estimate the magnitude of spin–orbit coupling for arbitrary molecules and multiconfigurational wave functions are reviewed. The form-factor method is extended from the original singlet–triplet formulation into arbitrary multiplicities. A simplified version of the mean-field method (the partial two-electron method, P2E) is formulated and tested versus the full two-electron operator on a set of representative molecules. The change of the one and two-electron spin–orbit coupling down the Periodic Table is investigated, and it is shown that the computationally much less demanding P2E method has an accuracy comparable to that of the full two-electron method.