Two-dimensional isotropic system with a negative poisson ratio

A two-dimensional (2D) lattice model of hexagonal molecules on a triangular lattice interacting via a nearest-neighbour n-inverse-power site-site potential is shown to display an anomalous (negative) Poisson ratio at high densities when the anisotropy (non-convexity) of the molecules is substantial. It is suggested that such a behaviour may be observed in some real systems.