Evidence of a two-state picture for supercooled water and its connections with glassy dynamics

Abstract

The picture of liquid water as consisting of a mixture of molecules of two different structural states (structured, low-density molecules and unstructured, high-density ones) represents a belief that has been around for long time awaiting for a conclusive validation. While in the last years some indicators have indeed provided certain evidence for the existence of structurally different "species", a more definite bimodality in the distribution function of a sound structural quantity would be desired. In this context, our present work combines the use of a structural parameter with a minimization technique to yield neat bimodal distributions in a temperature range within the supercooled liquid regime, thus clearly revealing the presence of two populations of differently structured water molecules. Furthermore, we elucidate the role of the inter-conversion between the identified two kinds of states for the dynamics of structural relaxation, thus linking structural information to dynamics, a long-standing issue in glassy physics.

Understanding water’s anomalies with locally favoured structures

“…Note that the agreement holds to very high temperatures, and for all pressures, suggesting a strong microscopic basis for the relevance of our order parameter. This is in stark contrast to previous attempts to obtain a two-state description of water from microscopic information 37,44,45 , where the agreement was restricted to the deeply supercooled region. We note that most of previous models have unphysical saturations of the value of s at high temperature around 0.5 or even higher, unlike our model where so o1 at high temperatures (see also refs 25,26 for a review).…”

“…Several attempts have been made, each differing in the microscopic definition of the states involved. Examples include states based on ice polymorphs 39 , tetrahedral order 32,40,41 or relative distance between neighbours 37,[42][43][44][45] .…”

Local structure analysis in ab initio liquid water

“…Similar to the ab initio PBE0+TS-vdW(SC) simulations discussed above, the relative population of water molecules with I < 0.05Å 2 decreases by approximately 5% when the simulation temperature of the liquid is reduced from 330 K to 300 K using the TIP4P/Ice force field. Here, we note in passing that the dependence of P(I) on temperature has also been demonstrated with other classical water force fields [33][34][35] , yielding similar qualitative results to those observed in this work.…”

“…In this regard, recent studies have shown that the local structure index (LSI) 31,32 is a very insightful order parameter that can be employed to characterize the LDL and HDL molecular environments 23 . For instance, the distribution of the LSI order parameter was found to be bimodal in the corresponding inherent potential energy surface (IPES) of both supercooled and ambient liquid water [33][34][35] . However, no attempt has been made yet to analyze the local environment in liquid water based on structures (configurations) generated using the more sophisticated ab initio based potentials available within the framework of density functional theory (DFT).…”

Requirements of first-principles calculations of X-ray absorption spectra of liquid water

“…Several schemes to determine whether an H-bond is present or not have been suggested, but the purely geometrical definitions have thus far proven to be insufficiently sensitive. 7 A scheme that provides a bimodal distribution of LDL/HDL local structures, 51,52 when considering the inherent structure 53 of simulated water, is the local structure index 54 (LSI). This order parameter accounts for the structure and coordination of the first and second solvation shells.…”