PNO–CI (pair natural-orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. III. The molecules MgH₂, AlH₃, SiH₄, PH₃ (planar and pyramidal), H₂S, HCl, and the Ar atom

PNO–CI and CEPA–PNO calculations are performed for the molecules MgH₂, AlH, AlH₃, SiH₄, PH₃, H₂S, HCl, and the Ar atom. Two types of Gaussian basis sets are used; both sets contain one p-set on H. The “small” set includes one d-set on the heavy atom, the “standard” basis two d-sets and one f-set. Both for MgH₂ and Ar, a “large” and a “very large” basis are used as well, which contain additional polarization functions. The energy improvement due to the different polarization functions is analyzed. Hartree–Fock limits for the molecular energies are estimated. The computed valence shell correlation energies are analyzed in terms of quantities defined in part I, in particular in terms of the IEPA (independent electron pair) correlation energies ε_μ^IEPA and the error ΔE_IEPA of the IEPA scheme. Both the valence shell interorbital pair correlation energies and the IEPA error are smaller in absolute value than those of the corresponding first row hydrides, provided that one uses the localized representation. MgH₂, AlH₃, and SiH₄ are “good IEPA molecules”, i.e., ‖ΔE_IEPA‖ is unusually small for them. For Ar, calculations that include the L and M shell, and the K, L, and M shell correlation energy are reported. The best known variational energy of the Ar atom is obtained as −527.2592 a.u., the (nonvariational) CEPA energy being −527.2916 a.u. For the unknown molecules MgH₂ and AlH₃, binding energies, equilibrium geometries, and symmetric-stretching force constants are predicted. MgH₂ has a binding energy of 104 kcal/mol referred to Mg+2H; it is hence expected to be unstable with respect to Mg+H₂. The predicted binding energy of AlH₃ is ? 205 kcal/mol referred to Al+3H. The inversion barrier of PH₃ which amounts to ? 38 kcal/mol on SCF level is reduced by electron correlation to ? 35 kcal/mol.