Accurate and efficient treatment of spin-orbit coupling via second variation employing local orbitals

A method is presented that allows for efficient evaluation of spin-orbit coupling (SOC) in density-functionaltheory calculations. In the so-called second-variational scheme, where Kohn-Sham functions obtained in a scalarrelativistic calculation are employed as a basis for the spin-orbit-coupled problem, we introduce a rich set of local orbitals as additional basis functions. Also, relativistic local orbitals can be used. The method is implemented in the all-electron full-potential code EXCITING. We show that, for materials with strong SOC effects, this approach can reduce the overall basis-set size and thus computational costs tremendously.