PNO-Cl (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

R. Ahlrichs, F. Keil, H. Lischka, W. Kutzelnigg, V. Staemmler

Research output: Contribution to journalArticlepeer-review

Abstract

PNO-Cl and CEPA-PNO calculations are performed for the molecules MgH 2, AlH, AlH3, SiH4, PH3, H 2S, 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 εμIEPA and the error ΔEIEPA 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., |ΔEIEPA| 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 bamer of PH3 which amounts to ≈ 38 kcal/mol on SCF level is reduced by electron correlation to ≈ 35 kcal/mol.

Original languageEnglish
Pages (from-to)455-463
Number of pages9
JournalThe Journal of Chemical Physics
Volume61
Issue number3
StatePublished - 1974

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