A study of the relative importance of one and two-electron contributions to spin-orbit coupling

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. (C) 2000 American Institute of Physics. [S0021-9606(00)30107-6].