VBDFT(s): A Hückel-type semi-empirical valence bond method scaled to density functional energies. Application to linear polyenes

A semi-empirical valence bond (VB) method, called VBDFT(s), is described and applied to C_nH_n+2 conjugated hydrocarbons. The method is a Hückel-type VB scheme with energies scaled to density functional theory energies based on a single parameter; the matrix element λ due to spin transposition between bonded atoms. Total energies, excitation energies, resonance and π-bond energies are presented for the various species. The insight that can be derived from the corresponding wavefunctions is discussed.