Ab initio molecular orbital calculations on alpha-quartz

The calculation of properties of solids using SCF MO theory has been generally recognized as being difficult, because of the large numbers of atoms which are required to produce a credible representation of the system. Using the extremely powerful lCAP1 and lCAP2 parallel systems at IBM Kingston, it was possible to perform a large number of ab initio calculations, using a 3-21G basis, on an Si5016 12- model 'molecule' surrounded by 956 point ions to simulate the alpha-quartz lattice. Calculations were carried out to investigate the effect of altering the charge on the point ions - which were positioned at the lattice sites determined using X-ray diffraction, and accurately reproduced the electrostatic fields of alpha-quartz in the SCF MO treated region. The effect of truncating the Si5016 'molecule' with hydrogen atoms was also investigated. Once an optimum cluster had been determined, a series of calculations to investigate the geometry and defect binding energies of radiation induced defects in quartz were carried out. The results of these calculations show general agreement with experimental results and previous theoretical investigation of radiation damage in alpha-quartz