Session A57: Physics of Liquids I

Ab initio molecular dynamics, machine learning and complex liquids

Using liquid water as an example I will show that deep neural networks allow us to greatly accelerate ab-initio molecular dynamics simulations without loss of accuracy. The approach naturally leads to models with different levels of coarse-graining both for the electronic ground-state and the atomic structure information. The basic procedure was outlined in two recent papers [1,2].

[1] L. Zhang, J. Han, H. Wang, R. Car, W. E, Physical Review Letters 120, 14301 (2018)
[2] L. Zhang, J. Han, H. Wang, R. Car, W. E, The Journal of Chemical Physics 149, 034101 (2018)
  

A viscoelastic hydrodynamic theory of density fluctuations in liquids

Long-wavelength longitudinal phonons can propagate in liquids, but whether transverse phonons exist in liquids has been long debated. Moreover, according to the classic hydrodynamic theory, it was believed that the transverse modes have no causal relation to the density fluctuations. However, these points of view are challenged by the recent data measured in the generalized hydrodynamic regime from computer simulations and scattering experiments. Hence, in this work, a viscoelastic hydrodynamic approach is proposed to describe the density fluctuations of supercooled liquids in the generalized hydrodynamic regime. Based on the generalization of the Navier-Stokes stress-strain constitutive relation, viscoelastic temporal response and spatial anisotropic effect are integrated into the framework of the hydrodynamic theory using a time-dependent relaxation tensor. As a result, it is found that liquids may exhibit either transverse excitations or transverse kinetic relaxations, depending on specific viscoelastic responses. Moreover, it is demonstrated that anisotropy naturally leads to a hybrid contribution of longitudinal dynamics and transverse dynamics to the density and current correlation functions.   

Study of the Correlated Molecular Dynamics in D2O using Inelastic Neutron Scattering

We report the real-space correlated molecular dynamics in heavy water (D₂O). Dynamic structure factor, S(Q, E), obtained from high-resolution inelastic neutron scattering data over large Q (momentum transfer) and E (energy transfer) is double-Fourier transformed to obtain the Van Hove function, which is a time-dependent pair-correlation function. A two-step relaxation is observed in the intermolecular dynamics of D₂O. Its relaxation time and amplitude depends on the temperature. By comparing the results with classical MD simulations of D₂O and inelastic x-ray scattering measurements of H₂O^[1][2], correlated dynamics of water molecules in ps time-scale is discussed.

References
1. T. Iwashita et al, Science Advances 3, e1603079 (2017).
2. Y. Shinohara et al, Phys. Rev. E 98, 022604 (2018).   

Elucidating the role of vapor-liquid interfaces on ice nucleation

Vapor-liquid interfaces are known to accelerate heterogeneous ice nucleation at their vicinity, in a phenomenon known as contact freezing [1]. Their impact on homogeneous nucleation, however, is more ambiguous, as the experimental evidence for surface freezing [2], or enhancement of homogeneous nucleation at free interfaces, is not conclusive. Computational investigations of surface freezing also predict conflicting qualitative behaviors for different water models [3-4]. It has, however, been suggested that contact and surface freezing are related, as if a free interface can enhance heterogeneous nucleation, it must also have the same effect on homogeneous nucleation. In this work, we use forward-flux sampling [5] to explore this relationship, and observe that tetrahedral liquids undergoing surface freezing will also exhibit accelerated heterogenous nucleation kinetics when the crystal-nucleating agent is very close to the free interface. We attribute this finding to instabilities induced at a free interface due to the proximity of the nucleating surface.

1- A. J. Durant, Geophys Res Lett, 32, L20814 (2005).
2- A. Tabazadeh, PNAS, 99, 15873 (2002).
3- A. Haji-Akbari, PCCP, 16, 25916 (2014).
4- A. Haji-Akbari, PNAS, 114, 3316 (2017).
5- A. Haji-Akbari, JCP, 149, 072303 (2018).   

Transient Structured Fluctuations in a 2D Liquid

Using the Aperture Cross Correlation Function as diagnostic, we find the existence of transient structured fluctuations in the liquid phase of a two-dimensional system with a repulsive, core-softened pair potential [1]. This system supports several hexagonal solid phases, a square solid phase, liquid and hexatic phases, and a quasi-crystalline phase with 12-fold symmetry. Our computer simulations suggest the surprising results that deep within the liquid region and approaching transition boundaries, the structured fluctuations favor only the same symmetries supported in the ordered phases of the system, i.e. three-fold, four-fold, and twelve-fold. To our knowledge, no theory to date suggests why such a system would not display local fluctuations of other symmetries well within the liquid phase.

[1] Z. Krebs, A. B. Roitman, L. M. Nowack, C. Liepold, Binhua Lin, and S. A. Rice, J. Chem. Phys, 149, 034503 (2018).   

A new scenario describing `water-like` thermodynamic anomalies - the double liquid-liquid critical point scenario

Thermodynamic `water-like' anomalies in liquids have been a long unsolved problem in terms of
origin and implications for nature of the liquid (and other similar states). The widely accepted thermodynamic scenario that describes this behaviour in water models and Stillinger-Weber (SW) potential silicon is the second (liquid-liquid) critical point (LLCP) scenario. In this work we present simulation evidence for a novel scenario in wide range of SW liquids. In this scenario the locus of liquid density anomalies (the density minimum and maximum) encloses a region of thermodynamic space with two liquid-liquid critical points connected by a first order transition. We scan the parameter space of the SW potential (tetrahedrality and ideal bond angle parameter) and observe limiting instances where density anomalies disappear. We observe the interactions of density anomaly with the compressibility anomaly, heat capacity anomaly and spinodal line and confirm they show thermodynamically consistent behaviour.   

Molecular Dynamics Simulations for Optical Kerr Effect of Liquid and Supercooled Water

The optical Kerr effect (OKE) spectroscopy measured with heterodyne detection (HD) is a useful tool to provide information regarding intermolecular vibrations and structural relaxations in liquid water. Recently, the measurements of the OKE spectroscopy have been extended to supercooled water [1]. Though the measured results can be well described by using a phenomenological model, the time-resolved OKE spectroscopy of liquid and supercooled water still need a comprehensive understanding. In this paper, we investigated the OKE nuclear response functions of this peculiar liquid and their reduced spectral densities by performing molecular dynamics simulations with the TIP4P/2005 water model [2]. The collective polarizability of water was computed via a dipolar induction scheme, which involves the intrinsic polarizability and the first-order hyperpolarizability tensor of water molecule. Our simulation results were consistent with the HD-OKE experimental observations. The effects due to the first-order hyperpolarizability of water molecule were examined.

[1] A. Taschin, P. Bartolini, R. Eramo, R. Righini, and R.Torre, Nature Commun. 4, 2401 (2013).
[2] P. H. Tang and T. M. Wu, J. Mol. Liq. 269, 38 (2018).   

Systematic investigations of Infrared spectra of liquid water by first-principles methods

The infrared spectroscopy (IR) is a powerful experimental technique to probe the dynamic dipole fluctuations of water molecules, which are strongly correlated through the H-bond network of liquid water. Here, we compute the infrared spectra of liquid water based on ab initio molecular dynamics simulations by using maximally localized Wannier functions as basis, in which both the generalized gradient approximations (GGA) level exchange-correlation (XC) functional and meta-GGA level strongly constrained and appropriately normed (SCAN) XC functionals are employed. For comparison, the vibration frequencies of the water vapor are computed as well. Our theoretical studies show that the theoretical predictions of spectral features are systematically improved by better liquid water structural predictions through the more physical description of H-bond strengths as well as the capture of intermediate range van der Waals interactions by the SCAN meta-GGA functional.
  

Emergence of local slow dynamics of water molecules induced by sodium chloride

We report on the real-space correlated motion of water molecules in sodium chloride aqueous solution. Measurements of high-resolution inelastic X-ray scattering spectra, S(Q, E), over wide energy transfer (E) and momentum transfer (Q) enabled us to determine the Van Hove function, G(R, t), by Fourier transform of S(Q, E) over Qand E. The results show that the addition of sodium chloride induces the emergence of a slow decay component at the first neighbor peak in the Van Hove function. The intensity of the slow decay and that of the fast decay are proportional and inversely proportional to the salt molality, respectively, whereas their decay time is independent of the molality. This indicates that the slowed correlated dynamics of water molecules is localized in the hydrated shell around ions whereas the dynamics of bulk water is not influenced by the salt.