Spin-Orbit Coupling in the ab initio Pseudopotential Framework
  We describe the implementation of total angular momentum dependent pseudopotentials in a plane wave formulation of density functional theory. Our approach thus goes beyond the scalar-relativistic approximation usually made in ab initio pseudopotential calculations and explicitly includes spin-orbit coupling. We outline the necessary extensions and compare the results to available all-electron calculations and experimental data. 
G. Theurich and N. A. Hill