Classical structures in modern valence bond theory

The results of some recent ab initio valence bond calculations, in which both structure coefficients and orbital forms are optimized, are analysed. The origin of structures in which the optimum orbitals are no longer atomic in character but instead delocalized, is traced back to the presence of certain symmetries in the wavefunction. When such symmetries exist it is possible to choose alternative linear combinations of the delocalized orbitals and to rewrite the wavefunction in terms of VB structures of classical form. The advantages of the classical structures are discussed in the context of a simple example — a square planar conformation of four hydrogen atoms.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday