Starting from the full microscopic Breit-Pauli or no-pair spin-orbit H
amiltonians, we have devised an effective one-electron spin-orbit Hami
ltonian in a well defined series of approximations by averaging the tw
o-electron contributions to the spin-orbit matrix element over the val
ence shell. In addition the two-electron integrals were restricted to
comprise only one-centre terms. The validity of these approximations h
as been tested on several palladium containing compounds. Excellent ag
reement of the matrix elements of the mean-field operator with corresp
onding full results is observed; deviations amount to a few cm(-1) in
absolute value or at most 0.2% on a relative scale. The newly defined
mean-field operator can thus safely be employed to evaluate spin-orbit
effects in transition metal containing compounds.