  Previous research has indicated the possible existence of a liquid-liquid
critical point (LLCP) in models of silica at high pressure. To clarify this
interesting question we run extended molecular dynamics simulations of two
different silica models (WAC and BKS) and perform a detailed analysis of the
liquid at temperatures much lower than those previously simulated. We find no
LLCP in either model within the accessible temperature range, although it is
closely approached in the case of the WAC potential near 4000 K and 5 GPa.
Comparing our results with those obtained for other tetrahedral liquids, and
relating the average Si-O-Si bond angle and liquid density at the model glass
temperature to those of the ice-like beta-cristobalite structure, we conclude
that the absence of a critical point can be attributed to insufficient
"stiffness" in the bond angle. We hypothesize that a modification of the
potential function to mildly favor larger average bond angles will generate a
LLCP in a temperature range that is accessible to simulation. The tendency to
crystallize in these models is extremely weak in the pressure range studied,
although this tendency will undoubtedly increase with increasing stiffness.

Soft Condensed Matter