BAND STRUCTURES AND MOLECULAR-PROPERTIES OF POLYMERS FROM FINITE CLUSTER CALCULATIONS WITH CYCLIC PERIODIC BOUNDARY-CONDITIONS

The use of a finite cluster model with cyclic periodic boundary conditions for studying molecular and electronic structures of polymers within the MNDO framework is critically examined. A simple procedure is proposed to determine the k values corresponding to each of the discrete orbital energies obtained from the calculations. Thus, band structures can be effectively obtained from finite cluster models. The rapid convergence of the computed results with respect to the number and size of clusters chosen is demonstrated on the basis of the geometries, heats of formation per monomer, and orbital energies at common k points for trans-polyacetylene (PA). The band structures computed for polyacetylene (PA), poly(p-phenylene) (PPP), and polyacene (PAC) are generally within 0.1 eV of those obtained from detailed band calculations and show no edge effects. Preferred geometries energies ionization potentials, band structures, and band gaps have also been computed for polymethinimine (PMI) and polycyclobutadiene (PCBD). In agreement with experiment and/or other calculations, the present method correctly reproduces the Peierls distortion in PA and PMI and the preference for the "benzenoid" structure of PPP, but PCBD and PAC are found to have moderate band gaps.