A molecular dynamics simulation of the vitreous silica surface

The molecular dynamics (MD) computer simulation technique was used to simulate the vitreous silica (v-SiO2) surface. A modified Born–Mayer–Huggins potential function was used in the simulation; periodic boundary conditions in two dimensions (x and y) only were used to create a thin slab with free surfaces in the z directions. Radial distribution functions and bond angle distributions for interior regions and for surface regions (within several angstroms of the surface) were evaluated. In accordance with our understanding of the v-SiO2 surface, the MD simulation generated a surface in which the oxygen atoms rather than the silicon atoms predominated at the outer surface. Nonbridging oxygen (NBO) as well as bridging oxygen were found at the surface. The NBO–Si interatomic spacing was found to be about 0.08 Å less than the normal bridging O–Si spacing and is in accordance with calculations made from spectroscopic data of defects in v-SiO2. Also an additional peak at 110° was observed in the Si–O–Si bond angle distribution for bonds within several angstroms of the surface. These smaller bond angles indicate the presence of the expected strained siloxane bonds which can occur at the surface. The results of this work are significant in that they indicate the applicability of using the molecular dynamics technique to accurately simulate the v-SiO2 surface.