Density and bond-orientational relaxations in supercooled water

TL;DR

This study uses molecular simulations to investigate the relaxation dynamics of density and bond-orientational order in supercooled water models, challenging previous hypotheses about the nature of the liquid-liquid phase transition.

Contribution

It provides evidence that density relaxes more slowly than bond-orientational order, contradicting prior assumptions about relaxation time scales in supercooled water models.

Findings

Density relaxes more slowly than bond-orientational order.

The observed behavior challenges previous hypotheses about phase transition artifacts.

Similar relaxation patterns are found across different water models.

Abstract

Recent computational studies have reported evidence of a metastable liquid-liquid phase transition (LLPT) in molecular models of water under deeply supercooled conditions. A competing hypothesis suggests, however, that non-equilibrium artifacts associated with coarsening of the stable crystal phase have been mistaken for an LLPT in these models. Such artifacts are posited to arise due to a separation of time scales in which density fluctuations in the supercooled liquid relax orders of magnitude faster than those associated with bond-orientational order. Here, we use molecular simulation to investigate the relaxation of density and bond-orientational fluctuations in three molecular models of water (ST2, TIP5P and TIP4P/2005) in the vicinity of their reported LLPT. For each model, we find that density is the slowly relaxing variable under such conditions. We also observe similar behavior in the coarse-grained mW model of water. Our findings therefore challenge the key physical assumption underlying the competing hypothesis.e find that density relaxes significantly faster than bond-orientational order, as incorrectly predicted by this competing hypothesis.