  We propose an approach for computing the Gibbs free energy difference between
phases of a material. The method is based on the determination of the average
force acting on interfaces that separate the two phases of interest. This
force, which depends on the Gibbs free energy difference between the phases, is
computed by applying an external harmonic field that couples to a parameter
which specifies the two phases. Validated first for the Lennard-Jones model, we
demonstrate the flexibility, efficiency and practical applicability of this
approach by computing the melting temperatures of sodium, magnesium, aluminum
and silicon at ambient pressure using density functional theory. Excellent
agreement with experiment is found for all four elements, except for silicon,
for which the melting temperature is, in agreement with previous simulations,
seriously underestimated.
