On the performance of molecular model core potential orbitals in spin-orbit and electron correlation studies

The role of improved parametrization and accurate basis sets in model core potentials was studied in calculations of the spin-orbit coupling constants (in PH, AsH, and SbH) and of the electron correlation effects (in P-2, As-2, and Sb-2). An effective method of identifying and removing the intruder quasi-core orbitals from the virtual orbital space was proposed in connection with the post-Hartree-Fock calculations. The results demonstrated that (a) the flexible valence basis sets allow evaluation of the spin-orbit effects without resorting to any scaling techniques and (b) the intruder quasicore orbitals, even if left imbedded in the virtual space, have negligible effect on the values of the electron correlation energy.