  We evaluate the phase diagram of the ``BKS'' potential [Van Beest, Kramer and
van Santen, Phys. Rev. Lett. 64, 1955 (1990)], a model of silica widely used in
molecular dynamics (MD) simulations. We conduct MD simulations of the liquid,
and three crystals (beta-quartz, coesite and stishovite) over wide ranges of
temperature and density, and evaluate the total Gibbs free energy of each
phase. The phase boundaries are determined by the intersection of these free
energy surfaces. Not unexpectedly for a classical pair potential, our results
reveal quantitative discrepancies between the locations of the BKS and real
silica phase boundaries. At the same time, we find that the topology of the
real phase diagram is reproduced, confirming that the BKS model provides a
satisfactory qualitative description of a silica-like material. We also compare
the phase boundaries with the locations of liquid-state thermodynamic anomalies
identified in previous studies of the BKS model.
