ABINITIO STUDIES OF SOLITON DEFECTS IN TRANSPOLYACETYLENE

We present results from geometry optimizations of charged solition defects in segments of transpolyacetylene. In particular, the effect of the counterion on the localization of the soliton defect is investigated. Using an ab initio Hartree-Fock method, we find that the soliton is considerably more localized in the presence of a single counterion than without the counterion present. We have taken the case of a single counterion to represent the situation at low doping levels. In order to simulate the high doping regime, we include the electrostatic interaction with soliton-counterion complexes on neighbouring chains. For this case we observe an increase of the soliton width, compared to the system with a single counterion. Finally, we investigate the effect of electron correlation on the dimerization of polyenes. Including second order Moller-Plesset corrections to the Hartree-Fock energy, we observe an increase in the length of the double bonds in the chain whereas the single bonds are unchanged. Thus, the dimerization is found to decrease due to electron correlation.