Interface free energy for two dimensional Ising models

The interface free energy of an Ising model with nearest neighbour interactions on a generalized square lattice is calculated using a simple method suggested by Müller-Hartmann and Zittartz. For certain values of the coupling constants this lattice can be made to be topologically equivalent to either a hexagonal or triangular lattice. The method of calculation bypasses the full bulk problem by considering only special interface configurations of the spins. It is shown that a large number of exactly known results for these different types of lattice can be obtained using this simple method.