ENERGIES OF STRAINED SILICA RINGS

The energies of strained two-member and three-member rings of SiO4 tet rahedra an calculated using models, based on continuous SiO? networks. These rings are considered to form highly reactive ''defect'' centers in vitreous SiO2 and at its surface. The calculations are based on a generalized gradient approximation to density-functional theory. and g ive strain energies of 1.23 and 0.25 eV for two- and three-member ring s, considerably smaller than those previously estimated from Hartree-F ock calculations applied to small hydrogen-terminated molecular models . Structural results are compared with experiment for solids and molec ules containing such rings. Changes in bond charge densities due to ri ng strain are illustrated, and modifications of the electronic states of relaxed SiO2 networks caused by strained ring defects are discussed .