THE NATURE OF MULTIPLE BONDS .2. SIGNIFICANCE OF THE PERFECT-PAIRING APPROXIMATION

We examine the importance in describing a multiple bond of including n on-perfect-pairing spin couplings among the electrons. We describe how to relax the perfect-pairing restriction of the usual strongly orthog onal perfect-pairing approximation to the generalized valence bond met hod, while rigorously remaining within an independent-particle model t hat allows an orbital interpretation of the bonding. In calculations o n ethylene and the nitrogen molecule, we find that the contribution of non-perfect-pairing spin couplings to the description of multiple bon ds is remarkably small and that the perfect-pairing spin restriction i s a relatively benign limitation on the wave function, for molecular g eometries near the equilibrium geometry.