Relativistic MCSCF by means of quasidegenerate direct perturbation theory. I. Theory

Relativistic MC-SCF (multiconfiguration self-consistent field) in terms of quasidegenerate direct perturbation theory (DPT) of relativistic effects is formulated based on a recently presented theory of effective Hamiltonians for electrons in a model space. The appropriately defined diagonal and nondiagonal parts of operators play a key role in this context. Their definition is based on stationary conditions for the MC-SCF wave function. The formalism starts from nonrelativistic MC-SCF theory. The leading relativistic correction appears as an expectation value in terms of the nonrelativistic MC-SCF function, while the higher-order relativistic corrections require a coupled-MC-SCF type approach. (C) 2000 American Institute of Physics. [S0021-9606(00)31008-X].