  We investigate ground state configurations of atomic systems in two
dimensions interacting via short range pair potentials. As the number of
particles tends to infinity, we show that low-energy configurations converge to
a macroscopic cluster of finite surface area and constant density, the latter
being given by the density of atoms per unit volume in the triangular lattice.
In the special case of the Heitmann-Radin sticky disc potential and exact
ground states, we show that the macroscopic cluster has a (unique) Wulff shape.
This is done by showing that the atomistic energy, after subtracting off a bulk
part and re-scaling, Gamma-converges to a macroscopic anisotropic surface
energy.
