Nature of the anomalies in the supercooled liquid state of the mW model of water

TL;DR

This study investigates the anomalous thermodynamic behavior of the supercooled mW water model, comparing two theoretical scenarios—mixture and weak crystallization—to explain observed properties without supporting a liquid-liquid critical point.

Contribution

The paper demonstrates that both the mixture and weak crystallization models can reproduce the anomalies of the mW water model, highlighting limitations of weak crystallization theory in this context.

Findings

Both models accurately reproduce thermodynamic anomalies.

Weak crystallization coupling constants are too large, challenging the theory.

The mixture model aligns with the low-density molecular fraction predictions.

Abstract

The thermodynamic properties of the supercooled liquid state of the mW model of water show anomalous behavior. Like in real water, the heat capacity and compressibility sharply increase upon supercooling. One of the possible explanations of these anomalies, the existence of a second (liquid-liquid) critical point, is not supported by simulations for this model. In this work, we reproduce the anomalies of the mW model with two thermodynamic scenarios: one based on a non-ideal "mixture" with two different types of local order of the water molecules, and one based on weak crystallization theory. We show that both descriptions accurately reproduce the model's basic thermodynamic properties. However, the coupling constant required for the power laws implied by weak crystallization theory is too large relative to the regular backgrounds, contradicting assumptions of weak crystallization theory. Fluctuation corrections outside the scope of this work would be necessary to fit the forms predicted by weak crystallization theory. For the two-state approach, the direct computation of the low-density fraction of molecules in the mW model is in agreement with the prediction of the phenomenological equation of state. The non-ideality of the "mixture" of the two states never becomes strong enough to cause liquid-liquid phase separation, also in agreement with simulation results.