Electron Correlation and Separated Pair Approximation in Diatomic Molecules. II. Lithium Hydride and Boron Hydride

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Abstract
By use of the variational principle, nearly optimal antisymmetrized products of separated geminals have been determined for lithium hydride, boron hydride, and their separated atoms. The geminals are determined in terms of their natural orbitals, which are represented as expansions of Slater‐type atomic orbitals. By comparison of the fully optimized wavefunctions of the hydrides at their equilibrium distances with the corresponding wavefunctions for the separated atoms, it is shown that the inner shells are essentially invariant upon molecule formation and that the detailed correlation structure of the inner geminal can be neglected for the determination of difference properties. From a partitioning of the geminal expansions into a principal part and a correlation part, a corresponding partitioning is obtained for the total wavefunction and the energy. With the help of this partitioning the detailed correlation interactions are analyzed and the sources of intrapair correlations are exhibited. Comparisons are made with previous calculations in terms of this analysis. A method is developed for obtaining potential curves from many‐parameter wavefunctions in which the wavefunction is reoptimized at a few selected values of the internuclear separation. Moreover, at these few points, the correlating natural orbitals of the inner shells can be omitted during the course of the minimization. From the fact that the parameters are smooth and slowly varying functions of the internuclear distance, an interpolation scheme is proposed for determining the entire potential curve. From these potential curves the spectroscopic constants are calculated on the basis of the Dunham analysis.