  We present the results of a classical molecular dynamic simulation as well as
of an ab initio molecular dynamic simulation of an amorphous silica surface. In
the case of the classical simulation we use the potential proposed by van Beest
et al. (BKS) whereas the ab initio simulation is done with a Car-Parrinello
method (CPMD). We find that the surfaces generated by BKS have a higher
concentration of defects (e.g. concentration of two-membered rings) than those
generated with CPMD. In addition also the distribution functions of the angles
and of the distances are different for the short rings. Hence we conclude that
whereas the BKS potential is able to reproduce correctly the surface on the
length scale beyond approx 5 Angstroems, it is necessary to use an ab initio
method to predict reliably the structure at small scales.
