Local structure analysis in ab initio liquid water

Abstract

Within the framework of density functional theory, the inclusion of exact exchange and non-local van der Waals/dispersion (vdW) interactions is crucial for predicting a microscopic structure of ambient liquid water that quantitatively agrees with experiment. In this work, we have used the local structure index (LSI) order parameter to analyze the local structure in such highly accurate ab initio liquid water. At ambient conditions, the LSI probability distribution, P(I), was unimodal with most water molecules characterized by more disordered high-density-like local environments. With thermal excitations removed, the resultant bimodal P(I) in the inherent potential energy surface (IPES) exhibited a 3:1 ratio between high-and low-density-like molecules, with the latter forming small connected clusters amid the predominant population. By considering the spatial correlations and hydrogen bond network topologies among water molecules with the same LSI identities, we demonstrate that the signatures of the experimentally observed low-(LDA) and highdensity (HDA) amorphous phases of ice are present in the IPES of ambient liquid water. Analysis of the LSI autocorrelation function uncovered a persistence time of ∼ 4 ps-a finding consistent with the fact that natural thermal fluctuations are responsible for transitions between these distinct yet transient local aqueous environments in ambient liquid water.

Specific Heat and Transport Functions of Water

“…Such an idea is supported by the following experimental evidence: (i) high-T bulk liquid water, if rapidly cooled at ambient pressure, becomes LDA without crystallization, so that LDA appears directly related to the liquid [21]; (ii) the HDA is connected with the liquid at high P, by the ice crystal melting increasing pressure (the two amorphous having small entropies can be considered smoothly connected with the liquid state). Just by taking into consideration the discontinuous LDA-HDA transitions, and the findings of an MD simulation, it has been proposed that liquid supercooled water is really polymorphic, with the two glass phases as their evolution at the dynamical arrest [22,23]. Such a liquid polymorphism can give the explanation for the water's response functional characteristics in the supercooled state by means of a possible liquid-liquid transition (LLT) and a liquid-liquid critical point (LLCP).…”

High and low density patches in simulated liquid water

“…These features of the S(q) are often attributed to the co-existence of water molecules forming ordered locally tetrahedral patches (low density liquid -LDL) in equilibrium with water molecules that participate in distorted hydrogen bond patches (high density liquid -HDL) 26 . This hypothesis, however, can only be indirectly inferred from experiments and has been the subject of scrutiny by a number of theoretical investigations of bulk liquid water 21,[27][28][29][30][31][32] .…”

Searching for crystal-ice domains in amorphous ices

“…In this regard, the structure of LDA is reminiscent of the local structure of Ih. By contrast, HDA has interstitial molecules populating the space between the first and the second hydration shells, thus acquiring distorted local configurations similar to liquid water configurations at ambient conditions [16,[22][23][24][25][26]. LDA and HDA also differ in terms of the hydrogenbond network (HBN).…”