  One method for computationally determining phase boundaries is to explicitly
simulate a direct coexistence between the two phases of interest. Although this
approach works very well for fluid-fluid coexistences, it is often considered
to be less useful for fluid-crystal transitions, as additional care must be
taken to prevent the simulation boundaries from imposing unwanted strains on
the crystal phase. Here, we present a simple adaptation to the direct
coexistence method that nonetheless allows us to obtain highly accurate
predictions of fluid-crystal coexistence conditions. We test our approach on
hard spheres, the screened Coulomb potential, and a 2D patchy-particle model.
In all cases, we find excellent agreement between the direct coexistence
approach and (much more cumbersome) free-energy calculation methods. Moreover,
the method is sufficiently accurate to resolve the (tiny) free-energy
difference between the face-centered cubic and hexagonally close-packed crystal
of hard spheres in the thermodynamic limit. The simplicity of this method also
ensures that it can be trivially implemented in essentially any simulation
method or package. Hence, this approach provides an excellent alternative to
free-energy based methods for the precise determination of phase boundaries.
