Role of Atomic Valence States in Molecular Energy Calculations

Ab initio calculations of molecular binding energies are examined critically in a basis of antisymmetrized products of approximate atomic state functions (the approximate composite functions of Moffitt). It is shown that much more accurate results may be expected if the binding energy is calculated relative to a suitably defined generalized valence state V of the dissociation products. The energy of this state V relative to the ground state of the dissociation products is estimated from atomic spectral data. This procedure leads directly to the intra-atomic correlation correction previously proposed by the author as a necessary modification of Moffitt's method of atoms in molecules. Detailed calculations are carried out for the ground states of the molecules LiH and BH, and for the ground and excited states of the benzene molecule (in the usual π-electron approximation). In each case the intra-atomic correlation correction leads to a substantial improvement in the accuracy of the results.