Ab initio and electron correlation corrected energy band structure of polymeric five-membered heterocycles

Ab initio SCF molecular calculations are reported for five-membered heterocycles, such as pyrrole, furane, thiophene and selenophene. Clusters containing one to five units are fully optimized and the geometrical parameters are used to describe the structure of the unit cell and translation parameters used in one-dimensional ab initio crystal orbital calculations. The electronic structure of each cluster is analyzed. The influence of the basis set on the results is reported for the cluster and crystal orbital calculations. For these systems, the bandgap is found to have a direct functional dependence on the ionization potential of the heteroatom in the ring. In addition, quasi-particle energy band structure calculations are carried out for polypyrrole and polythiophene using different basis sets. This inclusion of electron correlation results in a significant reduction of the bandgaps.