LINEAR-MUFFIN-TIN-ORBITAL METHOD FOR HELICAL POLYMERS - A DETAILED STUDY OF TRANSPOLYACETYLENE

Results of a detailed theoretical study of trans-polyacetylene are reported. In the first part, we present results on periodic chains which have been obtained with the first-principles, density-functional, full-potential linear-muffin-tin-orbital method for helical polymers. The lowest total energy is found for a structure with alternating carbon-carbon bond lengths in good agreement with experimental findings. Electronic band structures and densities of states are also presented. Moreover, photoelectron cross sections are determined and are used in calculating optical spectra, which are shown to agree well with experiments. A qualitative loss function is used in comparison with experimental electron-energy-loss spectra, and the results of Compton-scattering experiments are predicted. In the second part, we use the band structures, total energy, and Mulliken populations as functions of the structure in deriving a generalized, single-particle, Su-Schrieffer-Heeger model, which subsequently is used in studying solitons and polarons. It is found that solitons are stable distortions for the charged chains, and that the gap levels are placed about 0.1 eV from the midgap position. The lattice distortion, but not the electron or spin density, is more localized within the present model than with the Su-Schrieffer-Heeger model. Polarons, on the other hand, are found to be very shallow and most likely not stable.