  Computing phase diagrams of model systems is an essential part of
computational condensed matter physics. In this paper we discuss in detail the
interface pinning (IP) method for calculation of the Gibbs free energy
difference between a solid and a liquid. This is done in a single equilibrium
simulation by applying a harmonic field that biases the system towards
two-phase configurations. The Gibbs free energy difference between the phases
is determined from the average force that the applied field exerts on the
system. As a test system we study the Lennard-Jones model. It is shown that the
coexistence line can be computed efficiently to a high precision when the IP
method is combined with the Newton-Raphson method for finding roots.
Statistical and systematic errors are investigated. Advantages and drawbacks of
the IP method are discussed. The high pressure part of the temperature-density
coexistence region is outlined by isomorphs.
