Benzene in the extended hückel approximation: an unexpected geometry

The equilibrium geometry predicted by the extended Hückel method for benzene with D_6h symmetry is found to correspond to a saddle point on the potential surface. The search for a more stable D_3h structure leads to the prediction that benzene is unstable in the EHT approximation. It is concluded that EHT calculations of molecular conformation are much less than has apparently been previously recognized.