Quasidegenerate relativistic direct perturbation theory and effective
Hamiltonians for electrons
Werner Kutzelnigg
Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780 Bochum
The construction of an effective Hamiltonian for quasidegenerate direct perturbation theory of
relativistic effects is presented. It is based on a transformation of the Dirac Hamiltonian, that
decouples both electronic from positronic states and states inside the model space from those
outside of it. The formalism of Rutkowski and Schwarz is contained in the new theory as a
special case. A generalization to infinite order, i.e. a non-perturbative iterative approach is
also possible. The new formalism is particularly suited for relativistic perturbation theory
applied to the MC-SCF method. Some preliminary results are presented.