Spin–orbit coupling with model core potentials

A method has been proposed to compute one- and two-electron spin–orbit coupling using model core potentials generated with spin-free Hamiltonian without any need for separately producing either spin-dependent model core potentials or use of some fitted parameters, such as effective charges. The use of approximations (one-centre spin–orbit integrals and partial two-electron contribution) is verified to work within 2% accuracy compared to much costlier full results. The new method has been tested on atoms, hydrides, and homonuclear diatomic cations of P, As, and Sb. The accuracy has been found to be 3% or better within the tested set.