No abstract

Using a recently proposed procedure, the localized molecular orbitals are determined for minimal-basis-set LCAO—SCF wavefunctions of the molecules LiH, BH, NH, FH, Li2, B2, Be2, N2, F2, CO, BF, LiF. In all cases, well-defined inner-shell, lone-pair, and bond orbitals are found. They exhibit a considerable degree of transferability between different molecules, and the s-p hybridization varies regularly across the periodic table. In some cases the localized molecular orbitals deviate from accepted intuition; in all cases the exact determination of intrasymmetry localization is important. A close relation is found between the localized molecular orbitals and those molecular orbitals which extremize the MO-overlap populations. In the case of the HF molecule, the localized orbitals are also determined for the exact self-consistent-field wavefunction and, the transformation from canonical to localized molecular orbitals is found to be almost identical with that in the minimal-basis-set approximation. Implications of the localization procedures and its results are discussed.

The use of pseudonatural orbitals (PNO) is proposed to improve the rate of convergence in the superposition of configurations (SOC). Natural orbitals are determined for selected electron pairs in the Hartree—Fock field of the n−2 electron core and are then used as the basis for the total SOC calculation. Since these natural orbitals are not natural for the n-electron system they are considered false or pseudonatural orbitals when used in the n-electron problem. The PNO basis has been applied to He2+ and H3 to test the convergence. Complete results are reported here only for He2+. The PNO's are quite successful in speeding up the convergence of the SOC and rendering the calculation of correlation energy quite practical in general. Gaussian-type orbitals (GTO) are used throughout and were not a serious impediment to obtaining quantitative accuracy. In fact the large number of unoccupied Hartree—Fock orbitals consequent upon the use of a GTO basis permit a straightforward determination of the PNO orbitals.

No abstract