Configuration Interaction in the Simple Valence-Bond Wavefunction for the Potential-Energy Surfaces of Sigma-Bonded Four-Center Exchange-Reaction Complexes

The two closed-shell and nine open-shell symmetry-adapted MO electron configurations contained in the simple nonionic VB wavefunction for H4 in the limit of 2H2 with D2h symmetry and in a D4h complex are identified and their coefficients are obtained in terms of overlap integrals. Numerical comparison of the configuration coefficients obtained from the nonionic VB wavefunction with those obtained by the full CI calculation of Wilson and Goddard show that the ionic contribution to the D4h H4 complex of side 2.54a0 is small in the ground state but significant in the upper state. The implications of a VB or CI treatment of four-center potential-energy surfaces regarding a recent extension of the Woodward–Hoffmann rules (conservation of orbital symmetry) to the reactions H2 + D2→2HD and H2 + I2→2HI are discussed.