  We conduct extensive molecular dynamics computer simulations of two models
for liquid silica [the model of Woodcock, Angell and Cheeseman, J. Phys. Chem.
65, 1565 (1976); and that of van Beest, Kramer and van Santen, Phys. Rev. Lett.
64, 1955, (1990)] to determine their thermodynamic properties at low
temperature T across a wide density range. We find for both models a wide range
of states in which isochores of the potential energy U are a linear function of
T^(3/5), as recently proposed for simple liquids [Rosenfeld and P. Tarazona,
Mol. Phys. 95, 141 (1998)]. We exploit this behavior to fit an accurate
equation of state to our thermodynamic data. Extrapolation of this equation of
state to low T predicts the occurrence of a liquid-liquid phase transition for
both models. We conduct simulations in the region of the predicted phase
transition, and confirm its existence by direct observation of phase separating
droplets of atoms with distinct local density and coordination environments.
