Numerical method for determining the interface free energy.

We propose a general method (based on the Wang-Landau algorithm) to compute numerically free energies that are obtained from the logarithm of the ratio of suitable partition functions. As an application, we determine with high accuracy the order-order interface tension of the four-state Potts model in three dimensions on cubic lattices of linear extension up to L=56. The infinite volume interface tension is then extracted at each β from a fit of the finite volume interface tension to a known universal behavior. A comparison of the order-order and order-disorder interface tension at β(c) provides a clear numerical evidence of perfect wetting.