Correspondence Between the Phase Diagrams of TIP5P Water and a Spherically Symmetric Repulsive Ramp Potential

TL;DR

This study compares the phase diagrams of TIP5P water and a simple ramp potential model, showing their anomalous behaviors are closely related through a specific mapping, shedding light on water-like anomalies.

Contribution

It demonstrates a correspondence between water's phase diagram and a simplified ramp potential model, revealing underlying mechanisms of anomalies.

Findings

Regions of anomalous behavior are mappable between models.

A single ramp particle corresponds to two TIP5P molecules.

Water-like anomalies arise from shell migration during compression or heating.

Abstract

We perform molecular dynamics simulations of a well-known water model (the TIP5P pair potential) and a simple liquid model (a two-scale repulsive ramp potential) to compare the regions of anomalous behavior in their phase diagrams. We select the parameters of the ramp potential by mapping it to an effective pair potential derived from the TIP5P 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 to two TIP5P molecules. We present heuristic arguments supporting point (ii). We also argue that the water-like 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.