Equilibrium crystal shape of hard squares with diagonal attractions

The author exactly calculates the anisotropic interfacial tension of a square lattice gas model with nearest-neighbour exclusion and next-nearest-neighbour attractions. This is achieved by a method which introduces the shift operator into the standard transfer matrix argument. From the calculated anisotropic interfacial tension the equilibrium crystal shape (ECS) is derived by the use of the Wulff construction. The ECS is a closed curve in the X-Y plane, represented as cosh( Lambda (X+Y)/kBT)+A3cosh( Lambda (X-Y)/kBT)+A4/2=0 with a scale factor Lambda and constants A3,A4. He argues that this shape is a universal one which appears as the ECSS of a wide class of models.