Jeff Armstrong

Water polarization induced by thermal gradients: The extended simple point charge model (SPC/E)

We investigate the non-equilibrium response of extended simple point charge (SPC/E) water to thermal gradients. Using non-equilibrium molecular dynamics simulations, we show that SPC/E water features the thermo-polarization orientation effect, namely, water becomes polarized as a response to a thermal gradient. The polarization field increases linearly with the thermal gradient, in agreement with predictions of non-equilibrium thermodynamics theory. This observation confirms the generality of the thermo-polarization effect, first reported using the Modified Central Force Model (MCFM), and shows this physical effect is present irrespective of the water model details, in particular, dipole moment magnitude and model flexibility. The magnitude of the effect is the same for both models, although the sign of the electrostatic field is reversed in going from the MCFM to the SPC/E model. We further analyze the impact that the molecular geometry and mass distribution has on the magnitude of the polarization. Our results indicate that the thermo-polarization effect should be observed in a wide range of polar fluids, including fluids where hydrogen bonding is not present. Using various molecular models, we show that the polarization of these fluids under appropriate thermodynamic conditions can be of the same order or stronger than in water.

Temperature inversion of the thermal polarization of water.

Temperature gradients polarize water, a nonequilibrium effect that may result in significant electrostatic fields for strong thermal gradients. Using nonequilibrium molecular dynamics simulations, we show that the thermal polarization features a significant dependence with temperature that ultimately leads to an inversion phenomenon, whereby the polarization field reverses its sign at a specific temperature. Temperature inversion effects have been reported before in the Soret coefficient of aqueous solutions, where the solution changes from thermophobic to thermophilic at specific temperatures. We show that a similar inversion behavior is observed in pure water. Microscopically, the inversion is the result of a balance of dipolar and quadrupolar contributions and the strong temperature dependence of the quadrupolar contribution, which is determined by the thermal expansion of the liquid.

Computational Verification of Two Universal Relations for Simple Ionic Liquids. Kinetic Properties of a Model 2:1 Molten Salt

Two semianalytical relations [Nature, 1996, 381, 137 and Phys. Rev. Lett. 2001, 87, 245901] predicting dynamical coefficients of simple liquids on the basis of structural properties have been tested by extensive molecular dynamics simulations for an idealized 2:1 model molten salt. In agreement with previous simulation studies, our results support the validity of the relation expressing the self-diffusion coefficient as a function of the radial distribution functions for all thermodynamic conditions such that the system is in the ionic (ie., fully dissociated) liquid state. Deviations are apparent for high-density samples in the amorphous state and in the low-density, low-temperature range, when ions condense into AB(2) molecules. A similar relation predicting the ionic conductivity is only partially validated by our data. The simulation results, covering 210 distinct thermodynamic states, represent an extended database to tune and validate semianalytical theories of dynamical properties and provide a baseline for the interpretation of properties of more complex systems such as the room-temperature ionic liquids.

Note: How does the treatment of electrostatic interactions influence the magnitude of thermal polarization of water? The SPC/E model

We investigate how the treatment of electrostatic interactions influences the magnitude of the thermal polarization of water. We performed non-equilibrium molecular dynamics simulations of the extended simple point charge model of water under a thermal gradient, using two different systems: a water droplet confined in a spherical wall where the interactions are computed exactly using the Coulombic potential and a periodic prismatic box using the Wolf and 3D Ewald methods. All the methods reproduce the thermal polarization (TP) of water as well as the direction of the TP field, but the standard implementation of the Wolf method overestimates the strength of the TP field by one order of magnitude, showing that this method might be problematic in simulations involving temperature and/or density gradients.

Enhancement of the Thermal Polarization of Water via Heat Flux and Dipole Moment Dynamic Correlations

It has been recently shown that liquid water polarizes as a response to a temperature gradient. This polarization effect can be significant for temperature gradients that can be achieved at micro and nanoscales. In this paper we investigate the dependence of the polarization response of liquid and supercritical water at different thermodynamic conditions using both equilibrium and nonequilibrium molecular dynamics simulations for the extended point charge water model. We find that the thermal polarization features a nonmonotonic behavior with temperature, reaching a maximum response at specific thermodynamic states. We show that the thermal polarization is maximized when the density of states of the heat flux and dipole moment correlation functions feature the strongest overlap. The librational modes of water are shown to play an important role in determining this behavior as well as the heat transport mechanism in water. The librational frequencies show a significant dependence with temperature and pressure. This dependence provides a microscopic mechanism to explain the observed maximization of the thermal-polarization effect. Our work provides new microscopic insights on the mechanism determining the orientation of polar fluids under thermal gradients, as well as new strategies to maximize their orientation by manipulating the dynamic correlations between the heat flux and the sample dipole moment.

Chapter 6:Non-equilibrium Molecular Dynamics

This chapter discusses non-equilibrium molecular dynamics computer simulations. The focus is on the computation of coefficients that quantify transport properties (diffusion, thermal conductivity and viscosity) of simple and complex fluids as well as their interfaces. Several non-equilibrium methods are discussed, highlighting their connection to non-equilibrium thermodynamics. Coupling phenomena are also considered and applications to bulk fluids and interfaces are reviewed.

Molecular (and Lattice) Dynamics to Analyse Neutron Scattering Experiments 2016—MDANSE2016

Neutron News Volume 28 • Number 1 • 2017 17 November 2016 hailed the fi rst journey across the English Channel of Molecular (and Lattice) Dynamics to Analyze Neutron Scattering Experiments—MDANSE2016. This event was jointly organized by the ILL in Grenoble and ISIS at the Rutherford Appleton Laboratory, cementing a long-lasting partnership between the two neutron facilities. The Cosener’s House in Abingdon, just a stone’s throw away from ISIS, was the perfect location for its fi rst visit to British shores. The event aimed to bridge the knowledge gap between scientists coming from either experimental or computational backgrounds. With 48 delegates, it was a sell-out, attracting a balanced mix of attendees from either side of La Manche. The three-day meeting focused on the two main computational methodologies currently in use to analyze neutron-spectroscopy experiments: Molecular Dynamics (MD) and Lattice Dynamics (LD), both of which can be driven via the use of traditional force fi elds or state-ofthe-art electronic-structure calculations, most notably those based on Density Functional Theory (DFT). The formal lectures on day one focused on these two methods and were given by both members of the ILL and ISIS. Felix Fernandez-Alonso from ISIS kicked off proceedings, welcoming all delegates and highlighting the critical importance of in-silico methods for neutron spectroscopy in a modern era of increasingly complex materials. Stewart Parker, ISIS catalysis expert and champion of the UK Catalysis Hub at the Harwell Campus, gave a general overview of the how and the why neutron scattering is inextricably linked to computer simulations. Stewart was followed up by Miguel González from the ILL, who introduced MD and the use of force fi elds. The second set of afternoon talks during the fi rst day focused on ab-initio methods, with ISIS DFT gurus Sanghamitra Mukhopadhyay and Keith Refson giving an overview of electronicstructure methods and a follow-up lecture on how these are used to calculate vibrational spectra for direct comparison to experimental data, respectively. Delegates then enjoyed their fi rst meal of the event at the local and stylish restaurant The Crown and Thistle, before an early night in preparation for the practical sessions of the second day. The practical sessions were divided into the two main methodologies of MD and LD, and delegates were given the opportunity to choose the session which was most applicable to their research interests. These tutorials were lively and interactive, with experts from both facilities providing one-on-one tuition, and with ample time for the attendees to discuss and explore how these methods could be applied to their own research. An intense day of hands-on Molecular (and Lattice) Dynamics to Analyze Neutron Scattering Experiments 2016— MDANSE2016