Kekule subspace

The relevance of resonance-theoretic wave-functions naturally suggests at least one more derivative VB model, obtained by restriction of the covalent-space (Heisenberg) VB model to the subspace of (neighbor-paired) Kekule structures. In fact a whole hierarchy of models begins to emerge, as discussed a little more in the next section. 

Within the RVB scheme for the ground state, the simplest wave function beyond that of a single Kekule structure can be defined as a simple equally-weighted superposition of Kekule structures (EWK) within a given LR-SPO subspace,... 

For the linear chain there is only one Kekule structure in each LR-SPO subspace. Therefore, two main different ansatze including long SP may be considered. See, for instance, Refs. 24, 31 and 35. 

For the benzene molecule with six 7r-orbitals the 5 = 0 subspace is spanned by five basis states. A particular nonorthogonal representation of these basis states is illustrated in Fig. G.l. There are two equivalent Kekule structures and three equivalent Dewar structures (Coulson 1961). 

---