A COMPLETE ACTIVE SPACE VALENCE-BOND METHOD WITH NONORTHOGONAL ORBITALS

A complete active space self-consistent field (SCF) wave function is t ransformed into a valence bond type representation built from nonortho gonal orbitals, each strongly localized on a single atom. Nonorthogona l complete active space SCF orbitals are constructed by Ruedenberg's p rojected localization procedure so that they have maximal overlaps wit h the corresponding minimum basis set of atomic orbitals of the free-a toms. The valence bond structures which are composed of such nonorthog onal quasiatomic orbitals constitute the wave function closest to the concept of the oldest and most simple valence bond method. The method is applied to benzene, butadiene, hydrogen, and methane molecules and compared to the previously proposed complete active space valence bond approach with orthogonal orbitals. The results demonstrate the validi ty of the method as a powerful tool for describing the electronic stru cture of various molecules. (C) 1997 American Institute of Physics. [S 0021-9606(97)01647-4].