Resolving a puzzling anomaly in the spin-coupled generalized valence bond description of benzene.

In an earlier study of benzene, Small and Head-Gordon found that the spin-coupled generalized valence bond (SCGVB) wave function for the pi system predicted a distorted (non-D-6h) geometry, one with alternating CC bond lengths. However, the variations in the energy were very small and the predictions were made using a very small basis set (STO-3G). We re-examined this prediction using a much larger basis set (aug-cc-pVTZ) to determine the dependence of the energy of benzene on the distortion angle, Delta theta(CXC) (Delta theta(CXC) = 0 degrees corresponds to the D-6h structure). We also found a distorted geometry with the optimum Delta theta(CXC) being 0.31 degrees with an energy 0.040 kcal mol(-1) lower than that for the D-6h structure. In the optimum geometry, adjacent CC bond lengths are 1.3861 angstrom and 1.4004 angstrom. Analysis of the SCGVB wave function led us to conclude that the cause of the unusual non-D-6h geometry predicted by the SCGVB calculations seems to be a result of the interaction between the Kekule and Dewar components of the full SCGVB wave function. The addition of doubly ionic configurations to the SCGVB wave function leads to the prediction of a D-6h geometry for benzene and a dependence on Delta theta(CXC) essentially the same as that predicted by the complete active space self-consistent field wave function.