Cluster generalization of the dynamical coherent-potential approximation: Application to exciton-phonon interactions in molecular crystals

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Abstract
A cluster generalization of Sumi's dynamical coherent-potential approximation (DCPA) for the thermally averaged Green's function of interacting exciton-phonon systems is presented. Numerical solutions are obtained for a simple model Hamiltonian at 0 K and compared to the single-site DCPA and perturbative results. The Green's function is used to compute optical spectra, and it is shown that the bandwidth renormalization imposed by the two-site DCPA has significant effects in the intermediate-coupling regime.