Free energy surface of supercooled water

We present a detailed analysis of the free energy surface of a well characterized rigid model for water in supercooled states. We propose a functional form for the liquid free energy, supported by recent theoretical predictions [Y. Rosenfeld and P. Tarazona, Mel. Phys. 95, 141 (1998)], and use it to locate the position of a liquid-liquid critical point at T-C' = 130+/-5 K, P-C' = 290+/-30 MPa, and rho (C') = 1.10+/-0.03 g/cm(3). The observation of the critical point strengthens the possibility that the extended simple point charge model of water may undergo a liquid-liquid phase transition. Finally, we discuss the possibility that the approach to the liquid-liquid critical point could be pre-empted by the glass transition.