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

We present an ab initio investigation of the structural and electronic properties of the (0001) alpha-quartz surface. Five different models of this surface are generated by cleavage of the bulk followed by atomic relaxation and constant-temperature molecular dynamics. The most favorable reconstruction 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 surface is very similar to the bulk one, except for a typical feature of SiO2 under pressure, namely the disappearance of the gap between Si-O bonding and O 2p nonbonding states