Abstract
The use of semi-empirical quantum chemistry to calculate the exciton binding energy of conjugated polymers is discussed. Both the Pariser-Parr-Pople (PPP) model with Ohno parameterization and the models present in the MOPAC program overestimate the exciton binding energy relative to that observed in solid-state materials. Inclusion of Coulomb screening from adjacent chains may correct this overestimation. The solvation energy of a point charge in polyacetylene is calculated as 0.9 eV, using Hartree-Fock theory to describe the polarization induced in the solvent chains. It is argued that including screening by modifying the electron-electron interaction energy of the PPP model introduces physically unreasonable side effects and is not consistent with the 0.9-eV solvation energy of a point charge. Electron-hole screening models are then discussed along with the need to consider the relative time scales of the electron-hole motion and the dielectric response. (Author)