Ab initio calculations on valence-shell molecular orbitals

Abstract

A method of calculating valence-electron wavefunctions and energies in an SCF MO scheme without including inner shells in the basis is extended to first row transition metals and better than minimal basis sets. Orthogonalization of the valence basis to the atomic core eigenfunctions is shown to give better results than Schmidt-orthogonalization to the core Slater-type orbitals.

Calculations on CF₄, Al₂Cl₆, TiH₃F and MnO^–₄ are presented. For Al₂Cl₆ a factor of eight in computation time is saved over an equivalent all-electron calculation and a standard error of less than 1 eV in the energies of the occupied orbitals is obtained. Calculations on transition metal compounds are found to require the inclusion of 3s and 3p electrons in the valence basis for satisfactory results.

Neglect of overlap between the valence basis and core functions on adjacent centres is identified as the major source of error. This can be alleviated by careful choice of basis. The use of empirical one-centre nuclear attraction integrals in the calculations was investigated in a study of CF₄ but the predicted ionization potentials were not significantly closer to the observed values than were those obtained with non-empirical values for these integrals.