Using exact finite-size diagonalizations of extended Peierls-Hubbard Hamiltonians for systems of up to 15 sites, we study the excited state spectra of models for the finite oligomer analogs of trans– and cis– polyacetylene. We use a novel “phase randomization” technique and a variety of other methods to extract maximal information from these small systems. For several electron-phonon couplings in the expected range, we discuss the extent to which we are able to find consistent values of the Hubbard on-site (U) and nearest-neighbor (V) correlation parameters such that the band gap, the 2 1Ag state, optical phonon frequency, and the optical absorptions for charged and neutral solitons and dimers occur in the experimentally expected ranges. Our results suggest that these correlation parameters are in the intermediate coupling regime (U ⋍ 2.5t0), beyond the range of perturbation theory.
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For a recent review of the ePH Hamiltonian applied to (CH)x, see: D. Baeriswyl, D. K. Campbell, and S. Mazumdar, to be published in Conducting Polymers, edited by H. Kiess (Springer, New York, 1990).
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Gammel, J., Campbell, D.K., Loh, E.Y. et al. Determination of Correlation Parameters in π-Conjugated Polymers. MRS Online Proceedings Library 173, 419–424 (1989). https://doi.org/10.1557/PROC-173-419