Showing papers by "Roberto Car published in 2014"

Metastable liquid–liquid transition in a molecular model of water

Abstract: A stable crystal phase and two metastable liquid phases of the ST2 model of water exist at the same deeply supercooled condition, and the two liquids undergo a first-order liquid–liquid transition that meets stringent thermodynamic criteria Water's anomalous physical properties become markedly enhanced upon supercooling below the freezing point and even seem to diverge towards infinity at around 228 K Two papers in this issue use contrasting techniques to study this little-explored 'no-man's land' of water where extremely fast ice formation has prohibited measurements of the liquid state Jonas Sellberg et al use femtosecond X-ray laser pulses to measure bulk liquid water structure in droplets evaporatively cooled to 227 K Even at this temperature some droplets remained liquid on a millisecond timescale Pushing this technique further can shed light on controversial scenarios that aim to describe and explain the many anomalous properties of water Jeremy Palmer et al use six advanced computational methods to demonstrate the existence of two metastable liquid phases of ST2 water at the same deeply supercooled condition, undergoing a liquid–liquid transition that meets stringent thermodynamic criteria and could explain the behavior of water in this regime Liquid water’s isothermal compressibility1 and isobaric heat capacity2, and the magnitude of its thermal expansion coefficient3, increase sharply on cooling below the equilibrium freezing point Many experimental4,5,6,7,8, theoretical9,10,11 and computational12,13 studies have sought to understand the molecular origin and implications of this anomalous behaviour Of the different theoretical scenarios9,14,15 put forward, one posits the existence of a first-order phase transition that involves two forms of liquid water and terminates at a critical point located at deeply supercooled conditions9,12 Some experimental evidence is consistent with this hypothesis4,16, but no definitive proof of a liquid–liquid transition in water has been obtained to date: rapid ice crystallization has so far prevented decisive measurements on deeply supercooled water, although this challenge has been overcome recently16 Computer simulations are therefore crucial for exploring water’s structure and behaviour in this regime, and have shown13,17,18,19,20,21 that some water models exhibit liquid–liquid transitions and others do not However, recent work22,23 has argued that the liquid–liquid transition has been mistakenly interpreted, and is in fact a liquid–crystal transition in all atomistic models of water Here we show, by studying the liquid–liquid transition in the ST2 model of water24 with the use of six advanced sampling methods to compute the free-energy surface, that two metastable liquid phases and a stable crystal phase exist at the same deeply supercooled thermodynamic condition, and that the transition between the two liquids satisfies the thermodynamic criteria of a first-order transition25 We follow the rearrangement of water’s coordination shell and topological ring structure along a thermodynamically reversible path from the low-density liquid to cubic ice26 We also show that the system fluctuates freely between the two liquid phases rather than crystallizing These findings provide unambiguous evidence for a liquid–liquid transition in the ST2 model of water, and point to the separation of time scales between crystallization and relaxation as being crucial for enabling it

The individual and collective effects of exact exchange and dispersion interactions on the ab initio structure of liquid water

Abstract: In this work, we report the results of a series of density functional theory (DFT) based ab initio molecular dynamics (AIMD) simulations of ambient liquid water using a hierarchy of exchange-correlation (XC) functionals to investigate the individual and collective effects of exact exchange (Exx), via the PBE0 hybrid functional, non-local van der Waals/dispersion (vdW) interactions, via a fully self-consistent density-dependent dispersion correction, and an approximate treatment of nuclear quantum effects, via a 30 K increase in the simulation temperature, on the microscopic structure of liquid water. Based on these AIMD simulations, we found that the collective inclusion of Exx and vdW as resulting from a large-scale AIMD simulation of (H2O)128 significantly softens the structure of ambient liquid water and yields an oxygen-oxygen structure factor, SOO(Q), and corresponding oxygen-oxygen radial distribution function, gOO(r), that are now in quantitative agreement with the best available experimental data. This level of agreement between simulation and experiment demonstrated herein originates from an increase in the relative population of water molecules in the interstitial region between the first and second coordination shells, a collective reorganization in the liquid phase which is facilitated by a weakening of the hydrogen bond strength by the use of a hybrid XC functional, coupled with a relative stabilization of the resultant disordered liquid water configurations by the inclusion of non-local vdW/dispersion interactions. This increasingly more accurate description of the underlying hydrogen bond network in liquid water also yields higher-order correlation functions, such as the oxygen-oxygen-oxygen triplet angular distribution, POOO(θ), and therefore the degree of local tetrahedrality, as well as electrostatic properties, such as the effective molecular dipole moment, that are in much better agreement with experiment.

Response to Comment [arXiv:1407.6854] on Palmer et al., Nature, 510, 385, 2014

Abstract: We respond to a Comment [arXiv:1407.6854 (2014)] on our recent Nature paper [Nature, 510, 385 (2014)]. We categorically disprove the arguments provided in arXiv:1407.6854 (2014) and thereby further substantiate the evidence we presented in our recent study, demonstrating the existence of a metastable liquid-liquid transition in a molecular model of water. We will make our code publicly available shortly along with proper user documentation that is currently under development.