Conductivity in Polyacetylene. 3. <i>Ab</i> <i>Initio</i> Calculations for a Two-Site Model for Electron Transfer

Electron exchange between ethene molecules is studied using quantum chemical ab initio methods in order to model interchain electron transfer (ET) in polyacetylene. The potential surface for the system is obtained with the distance between the carbon atoms in each molecule as independent variables. The reaction path, reorganization energy for ET, λ, and the electronic factor, Δ, are calculated using the UHF and UMP2, CASSCF, CASPT2, and CASSI methods. It is found that the potential surfaces can be represented as interacting parabolic-well potentials only in the case of CASSI. The UHF, UMP2, CASSCF, and CASPT2 methods give localization of the electron even in the symmetric case and incorrect energy surfaces. The factor Δ calculated using the Koopmans' theorem in Hartree−Fock and CASSI are quantitatively very close for all intermolecular distances under study. However, Δ is sensitive to the basis set and cannot be calculated accurately in the present case.