A COMMENT ON THE CHARGE EXCITATION GAP FOR POLYACETYLENE

The charge excitation gap for polyacetylene (CH)x was calculated using the Lanczos algorithm applied to small chains, The actual value of 4. 5 +/- 0.4 eV for this gap was obtained using the extended Hubbard mode l with the parameters fixed from ab initio calculations (reported else where). For the second set of model Hamiltonian parameters recommended by many experimentalists (those measuring optical gaps or on-site spi n densities in polyacetylene) the corresponding value for the charge t ransfer gap is 1.6 +/- 0.8 eV, which is reasonably close to what is co nsidered the experimental value. Our results confirm the earlier findi ng that the electron correlations favour dimerization in polyacetylene .