The perturbation of a polymer molecular orbital

The symmetry restrictions imposed by the Born-Kármán boundary conditions on a polymer in order to calculate its electronic band structure are relaxed by the separate introduction of two types of perturbation at a site on the polymer chain. The first simulates a defect or the close approach of a molecule by modifying the segment orbital functions in the environment of the site while the second simulates a change in the distance between one pair of adjacent segments by altering the interaction between them. When applied to a molecular orbital described by a Bloch function and arbitrary energy band these perturbations generate a small number of electronic states whose energies lie above and below those of the band. The sign of the energy shift depends on the symmetry of the crystal orbital perturbed.