First-principles molecular-dynamics study of the (0001) alpha-quartz surface

We present an ab initio investigation of the structural and electronic prop erties of the (0001) alpha-quartz surface. Five different models of this su rface are generated by cleavage of the bulk followed by atomic relaxation a nd constant-temperature molecular dynamics. The most favorable reconstructi on presents an unexpected densification of the two uppermost layers of SiO2 tetrahedral units, with three-membered and six-membered rings that do not exist in bulk alpha-quartz. The electronic density of states for this surfa ce is very similar to the bulk one, except for a typical feature of SiO2 un der pressure, namely the disappearance of the gap between Si-O bonding and O 2p nonbonding states.