Minimum energy configurations of repelling particles in two dimensions

Geometrical arrangements of minimum energy of a system of identical repelling particles in two dimensions are studied for different forms of the interaction potential. Stability conditions for the triangular structure are derived, and some potentials not satisfying them are discussed. It is shown that in addition to the triangular lattice, other structures may appear (some of them with non-trivial unit cells, and non-equivalent positions of the particles) even for simple choices of the interaction. The same qualitative behavior is expected in three dimensions.