Correspondence between phase diagrams of the TIP5P water model and a spherically symmetric repulsive ramp potential with two characteristic length scales.

We perform molecular dynamics simulations of the TIP5P water model and derive the physical parameters for a simple two-scale repulsive ramp potential model. We find that the regions of anomalous behavior in the phase diagrams of both systems can be mapped onto each other if (i) pressure P and temperature T are replaced by T-T(C) and P-P(C), respectively, where (T(C),P(C)) are the coordinates of the liquid-liquid critical point of the corresponding system; and (ii) a single ramp particle corresponds effectively to two TIP5P molecules. We present heuristic arguments supporting point (ii). We also argue that the waterlike anomalies in the ramp potential are due to the ability of the particles to reproduce, upon compression or heating, the migration of water molecules from the second shell to its first shell.