Line of compressibility maxima in the phase diagram of supercooled water

We evaluate thermodynamic, structural, and transport properties from extensive molecular-dynamics computer simulations of the ST2 and TIP4P models of liquid water over a wide range of thermodynamic states. We find a line in the phase diagram along which the isothermal compressibility of the supercooled liquid is a maximum. We further observe that along this line the magnitude of the maximum increases with decreasing temperature. Extrapolation to temperatures below those we are able to simulate suggests that the compressibility diverges. In this case, the line of compressibility maxima develops into a critical point followed at lower temperature by a line of first-order phase transitions. The behavior of structural and transport properties of simulated water supports the possibility of a line of first-order phase transitions separating two liquid phases differing in density. We therefore examine the experimentally known properties of liquid and amorphous solid water to test if the equation of state of the liquid might exhibit a line of compressibility maxima, possibly connected to a critical point that is the terminus of a line of phase transitions. We find that the currently available experimental data are consistent with these possibilities.