SPIN SEPARATION IN THE REGULAR HAMILTONIAN APPROACH TO SOLUTIONS OF THE DIRAC-EQUATION

It is shown how by combining the ideas of the direct perturbation theory approach to the solution of the Dirac equation and the regular expansion as used in the Chang-Pelissier-Durand Hamiltonian one can derive a spin-free approximation to the Dirac equation that resembles a similar equation recently derived by Dyall, However, unlike that equation, the present approach is entirely free of singular operators even in the case of a potential containing an attractive Coulomb singularity such as encountered in atomic and molecular Dirac-Fock theory.