Electronic structure of highly conducting polymers

Abstract

Abstract Results of ab initio SCF LCAO crystal orbital calculations are presented for pure and doped polyacetylenes, polydiacetylene backbones, stacked TCNQ and TTF columns and for linear chains of (SNI)X. The structural parameters of five different polyacetylene chains were fully optimized at the Hartree-Fock level and the cis-trans isomerisation energy was calculated by taking into account interchain interactions in the 3D crystal including dispersion. Several one-particle properties are discussed both for symmetry adapted and symmetry breaking solutions (bond-and spin density waves, Peierls distortion) and the effect of electronic correlation on those quantities is demonstrated. The effect of n-type doping with Li and Na was investigated in both cis- and trans polymers. Besides charge transfer the impurity atoms fundamentally change the geometrical structure (transforming the Peierls distorted chains to equidistant ones) and substantially reduce the fundamental gap. Four different polydiacetylene backbones are also compared from the point of view of stability and their one-electron properties are discussed. Finally, the effect of randomly distributed hydrogen impurity in (SN)X on the electronic structure is investigated with the help of the CPA method.