Non-applicability for the Li+  H2 ion molecule system of an INDO potential energy surface or of the approximate monopole-transition moment long range force expressions

Abstract

A potential energy surface for the ion-molecule system Li+ + H2 was calculated by a semi-empirical LCAO-MO-SCF INDO procedure to check the results against the published accurate Hartree-Fock LCAO-MO-SCF calculation using a large Gaussian basis set. The INDO surface did not reproduce at all the accurate surface although this is a system which separates properly in the Hartree-Fock approximation and, thus, one in which single determinant wave functions should pick up all of the ion-induced dipole long-range attractive energy. Approximate monopole and transition moment expressions used previously for calculation of long-range forces involving π-electrons only were extended to general systems including σ electrons. These expressions involve knowing the eigenvalues and eigenvectors for the separated fragments. It was shown that use of INDO eigenvalues and eigenvectors in the approximate long-range expression do not lead to proper values for these intermolecular energies. These INDO calculations, however, are equivalent in concept and accuracy to the Pariser-Parr-Pople calculations of the π-electron systems whose results have been used in the same long-range expressions in order to calculate π interactions between molecules containing π-electrons. As a check whether it was the minimal Slater orbital basis of the INDO calculation which was causing the inaccuracies when these results were used in long-range calculations, non-empirical ab initio minimal STO basis were carried out for the system Li+–H2. While the energy differences between the separated fragments and the intermediate supermolecule resulting from this ab initio STO calculations gave an accurate ion-induced dipole energy, use of the corresponding eigenvalues and eigenvectors in the long-range expressions still did not lead to agreement with accurate interaction energies calculations from the Hartree-Fock surface. Examination of the eigenvalues and eigenvectors of the intermediate supermolecule Li+–H2 indicated the fundamental physical basis for the non-applicability of the approximate long-range expressions.