Determination of interaction parameters in peierls-hubbard models describing finite polyenes and polyacetylene

We study the extent to which finite polyenes and polyacetylene can be described by the extended Peierls-Hubbard Hamiltonian (ePHH) in an internally consistent manner. Using Lanczos exact diagonalization methods (LEDM) on small systems, coupled with a novel boundary condition (b.c.) averaging technique, we investigate the excited state spectra of these models for the finite oligomer analogs of trans-polyacetylene, (CH)_x. We search for sets of parameters describing the electron-phonon (e-p) and electron-electron (e-e) interactions which are consistent with all available experimental data on these systems, including the optical gap and bandwidth, the 2¹ A_g state, longitudinal optical phonon frequency ω_LO², triplet excitations, and the optical absorptions assigned to charged and neutral solitons and to polarons. We conclude that the e-p coupling is substantially weaker than that suggested by purely e-p models and that the e-e interaction parameters are in the intermediate coupling regime (U ∼ 2.5t₀), consistent with values deduced from the finite polyenes.