  We propose a computational method that simplifies drastically the inclusion
of spin-orbit interaction in density functional theory implemented on localised
atomic orbital basis sets. Our method is based on a well-known procedure for
obtaining pseudopotentials from atomic relativistic 'ab initio' calculations
and on an on-site approximation for the spin-orbit matrix elements. We have
implemented the technique in the SIESTA code, and we show that it provides
accurate results for the overall band structure and splittings of group IV and
III-IV semiconductors as well as for 5d metals.
