THE NATURE OF MULTIPLE BONDS .3. BENZENE, BENT BONDS, AND RESONANCE

The phenomenon of resonance, in the classical valence bond sense, is i nvestigated with use of explicitly correlated ab initio wave functions of the generalized valence bond form. For molecules exhibiting ''reso nance'', requiring more than a single classical bonding structure to d escribe the bonding, the wave function is constructed as the sum of te rms, each term being associated with a particular valence bond structu re. The interaction of these terms in a nonorthogonal configuration in teraction or ''structure interaction'' calculation results in a wave f unction lower in energy than provided by any of the terms associated w ith the individual structures. Although the wave function as a whole c annot be interpreted within an independent particle context, each of t he terms that compose it can; in this paper, we examine the nature of the bonding in such a system and probe the relative merits of a sigma, pi symmetry-restricted bond representation and a bent bond representa tion of the bonding. The results serve to support the conclusions made in previous papers of this series regarding the nature of the bonding in multiple bonds. Based on energetic considerations, the bent bond m odel serves as a better framework with which to describe the electroni c structure in systems exhibiting resonance than the sigma, pi bond mo del.