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

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Abstract
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 nondynamical correlation and relativistic effects in berylliumlike systems are carried out. For this sequence, it appears that the single-state first-order DPT is valid only for the nuclear charge up to 30 or so. Calculations for the spectroscopic constants of the molecules of homonuclear halogen dimers X 2 (X=F, Cl, Br, I, At) and TlX (X=F, Cl, Br, I) are performed at all-electron SCF (self-consistent field) and complete active space SCF levels, with the aim to demonstrate the efficiency of DPT by comparing with other relativistic calculations. Spin–orbit coupling affects the energies of the open-shell atoms and is taken into account for the binding energies.