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Non‐applicability for the Li + H 2 ion molecule system of an INDO potential energy surface or of the approximate monopole‐transition moment long range force expressions
Author(s) -
Kaufman Joyce J.,
Predny R.
Publication year - 2009
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560050829
Subject(s) - linear combination of atomic orbitals , ab initio , atomic physics , chemistry , dipole , eigenvalues and eigenvectors , electron , moment (physics) , basis set , range (aeronautics) , ion , ab initio quantum chemistry methods , atomic orbital , physics , quantum mechanics , molecule , materials science , composite material
A potential energy surface for the ion‐molecule system Li + + H 2 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 + –H 2 . 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 + –H 2 indicated the fundamental physical basis for the non‐applicability of the approximate long‐range expressions.