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The recently proposed ab initio effective Hamiltonian technique is extended to polymer calculations and applied to various conformations of polyacetylene (all-trans, cis-transoid, and trans-cisoid) and polydiacetylene (acetylenic and butatrienic backbones). Band structures, density of states (DOS), and XPS theoretical spectra are presented. Comparison of the band structures and DOS with those obtained by ab initio SCF (self-consistent-field) Hartree---Fock calculations of double zeta quality is excellent. The XPS theoretical spectrum for all-trans polyacetylene is in good agreement with experiment. In polyacetylene, it is found that the 1T bands are quite similar for all three backbone conformations, though the a bands differ significantly. For polydiacetylene, low ionization potentials are predicted-a few tenths of an eV larger than polyacetylene for the acetylenic backbone and a few tenths of an eV smaller than …