AB-INITIO AND ELECTRON CORRELATION CORRECTED ENERGY-BAND STRUCTURE OFPOLYMERIC 5-MEMBERED HETEROCYCLES

Ab initio SCF molecular calculations are reported for five-membered he terocycles, such as pyrrole, furane, thiophene and selenophene. Cluste rs containing one to five units are fully optimized and the geometrica l parameters are used to describe the structure of the unit cell and t ranslation parameters used in one-dimensional ab initio crystal orbita l calculations. The electronic structure of each cluster is analyzed. The influence of the basis set on the results is reported for the clus ter and crystal orbital calculations. For these systems, the bandgap i s found to have a direct functional dependence on the ionization poten tial of the heteroatom in the ring. In addition, quasi-particle energy band structure calculations are carried out for polypyrrole and polyt hiophene using different basis sets. This inclusion of electron correl ation results in a significant reduction of the bandgaps.