Fermion-propagator calculations of excitations in polyenes with the use of a Heisenberg (XYZ) Hamiltonian. II. Applications to large systems

A previously developed formalism for calculating excitation energies in polyenes using an effective-spin Hamiltonian is applied to large systems. The method involves only diagonalization of N×N matrices (N being the number of sites). A description of the excitation process in terms of local spin flippings is given and values of the triplet and doublet exciton gap for finite polyenes can be estimated. When applied to deformed chains with odd N, midgap states appear. Introduction of electronic correlation renders local ground-state spin densities which are in qualitative agreement with electron-nuclear double-resonance measurements. Excited-state spin densities are also discussed in some cases.