Relativistic MCSCF by means of quasidegenerate direct perturbation theory.II. Preliminary applications

The relativistic multiconfiguration self-consistent field (MCSCF) theory by means of (quasidegenerate) direct perturbation theory (DPT) is formulated and implemented through the leading order. Systematic investigations of non dynamical correlation and relativistic effects in berylliumlike systems are carried out. For this sequence, it appears that the single-state first-ord er DPT is valid only for the nuclear charge up to 30 or so. Calculations fo r the spectroscopic constants of the molecules of homonuclear halogen dimer s X-2 (X=F, Cl, Br, I, At) and TlX (X=F, Cl, Br, I) are performed at all-el ectron SCF (self-consistent field) and complete active space SCF levels, wi th the aim to demonstrate the efficiency of DPT by comparing with other rel ativistic calculations. Spin-orbit coupling affects the energies of the ope n-shell atoms and is taken into account for the binding energies. (C) 2000 American Institute of Physics. [S0021-9606(00)31108-4].