  We use ab initio density functional theory to study the equilibrium geometry
and electronic structure of few-layer grey arsenic. In contrast to the bulk
structure that is semimetallic, few-layer grey As displays a significant band
gap that depends sensitively on the number of layers, in-layer strain, layer
stacking and inter-layer spacing. A metal-semiconductor transition can be
introduced by changing the number of layers or the in-layer strain. We
interpret this transition by an abrupt change in the spatial distribution of
electronic states near the top of the valence band.
