Method

  All defects discussed here were symmetry constrained to C_1h or C_2v as appropriate. In every case symmetry was later breached and in most cases the defects relaxed back to the symmetrical structures; exceptions are discussed in the text. 113 atom clusters were used for initial results, but the final results discussed here were obtained using 148-152 atom clusters. The results were qualitatively identical, showing that they are independent of cluster size. In some cases, e.g. the (CH)_iO_4i, the final structure shows slight deviation from its overall C_2v symmetry; where relevant this has been discussed in the text.

The Kohn-Sham energy levels arising from the calculations give only an approximate estimate of donor and acceptor levels. This being so, it is difficult to conclude that an occupied level lying close to the conduction band of unoccupied levels is a shallow donor as opposed to a donor whose ionisation energy is a few tenths of an eV. It certainly appears in this work that certain levels are very close to the conduction band and will be described as shallow to distinguish them from deep mid-gap states possessed by other defects.