  We develop a methodology for the calculation of surface free energies based
on the probability distribution of a wandering interface. Using a simple
extension of the NpT sampling, we allow the interface area to randomly probe
the available space and evaluate the surface free energy from histogram
analysis and the corresponding average. The method is suitable for studying
systems with either continuous or discontinuous potentials, as it does not
require explicit evaluation of the virial. The proposed algorithm is compared
with known results for the surface tension of Lennard--Jones and Square Well
fluid, as well as for the interface tension of a bead--spring polymer model and
good agreement is found. We also calculate interfacial tensions of freely
jointed tangent hard sphere chains on athermal walls for a wide range of chain
lengths and densities. The results are compared with three different
theoretical approaches, Scaled Particle Theory, the Yu and Wu density
functional theory and an analytical approximation based on the latter approach.
Whereas SPT only yields qualitative results, the last two approaches are found
to yield very good agreement with simulations.
