Free energy cost of forming an interface between a crystal and its frozen version

Using a thermodynamic integration scheme, we compute the free energy cost per unit area, γ, of forming an interface between a crystal and a frozen structured wall, formed by particles frozen into the same equilibrium structure as the crystal. Even though the structure and potential energy of the crystalline phase in the vicinity of the wall is the same as in the bulk, γ has a nonzero value and increases with increasing density of the crystal and the wall. Investigating the effect of several interaction potentials between the particles, we observe a positive γ at all crystalline densities if the potential is purely repulsive. For models with attractive interactions, such as the Lennard-Jones potential, a negative value for γ is obtained at low densities. A qualitative explanation for the change of sign of γ when going from repulsive to attractive interactions, at low crystal densities, is suggested.