Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P57: Physics of Liquids IIFocus
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Sponsoring Units: GSOFT DCP GSNP Chair: Jeremy Palmer, University of Houston Room: LACC 518 |
Wednesday, March 7, 2018 2:30PM - 3:06PM |
P57.00001: On the structure of liquids and glasses: More order than expected Invited Speaker: Walter Kob The structure of liquids and glasses is usually characterized by means of the radial distribution function or the static structure factor. Computer simulations or confocal microscopy experiments allow also to access the bond angle distributions or the local connectivity of the atoms. However, all these quantities are basically one-dimensional in nature and hence it is hard to infer from them the real three dimensional structure of amorphous systems. As a consequence the structure of liquids and glasses are usually considered to carry little information for distances beyond the second/third nearest neighbor. In this talk I will show that this is not the case at all and that by considering simple three dimensional correlation functions one finds a surprisingly ordered arrangement of the particles even at significantly larger distances. This order grows quite quickly if the temperature is lowered, showing that amorphous systems are way more ordered than expected from the study of the usual two-point correlation functions. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P57.00002: Ab initio theory and modeling of water Mohan Chen, Hsin-Yu Ko, Richard Remsing, Marcos Calegari Andrade, Biswajit Santra, Zhaoru Sun, Annabella Selloni, Roberto Car, Michael Klein, John Perdew, Xifan Wu Water is of the utmost importance for life and technology. However, a genuinely predictive ab initio model of water has eluded scientists. We demonstrate that a fully ab initio approach, relying on the strongly constrained and appropriately normed (SCAN) density functional, provides such a description of water (PNAS 114, 10846-10851). SCAN accurately describes the balance among covalent bonds, hydrogen bonds, and van der Waals interactions that dictates the structure and dynamics of liquid water. Notably, SCAN captures the density difference between water and ice Ih at ambient conditions, as well as many important structural, electronic, and dynamic properties of liquid water. These successful predictions of the versatile SCAN functional open the gates to study complex processes in aqueous phase chemistry and the interactions of water with other materials in an efficient, accurate, and predictive, ab initio manner. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P57.00003: Ab Initio Simulations of Water Using Self-Consistent Hybrid Functionals Jeffrey Gustafson, Alex Gaiduk, Francois Gygi, Giulia Galli Dielectric-dependent hybrid functionals, in particular self-consistent hybrid (sc-hybrid) [1], were recently shown to predict the photoelectron spectra of water [2] and several solutions [3] in good agreement with experiments. However, the trajectories used for the spectra calculations were generated using either PBE or the PBE0 functional. We carried out simulations of water at ambient conditions using a sc-hybrid functional [4] to compute the structural and diffusion properties of the liquid. We found excellent agreement with experiment for the oxygen-oxygen correlation functional at the experimental equilibrium density and T= 300K. Other structural and dynamical properties such as diffusion coefficient, molecular dipole moments, and vibrational spectra, are also in good agreement with experiment. Our results indicate that the sc-hybrid functional accurately describes both the structural and electronic properties of liquid water. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P57.00004: Solvation Properties of Ions in Confined Aqueous Solutions from First Principles Molecular Dynamics Viktor Rozsa, Tuan Anh Pham, Eric Schwegler, Giulia Galli The physiochemical properties of solvated ions in extreme conditions, such as nanoconfinement, are critical to the engineering of novel energy technologies. In this work, we compare the properties of various alkali halide solutions in the bulk and under confinement in single-walled carbon nanotubes of 1.5 nm diameter, as obtained from first-principles molecular dynamics simulations [1]. In particular, we present an analysis of solvation structures and hydrogen bond characters as a function of ionic species and the degree of confinement. In addition, we discuss the self-diffusion of ions and water, as well as the local and global effects of ions on the structure and dynamics of liquid water. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P57.00005: Infrared Spectra of Liquid Water Studied by Ab initio Methods Jianhang Xu, Mohan Chen, Xifan Wu The infrared (IR) spectroscopy can probe the dynamic fluctuation of molecule dipoles which are strongly correlated by the H-bond network of liquid water. So far, the quantitative agreement with experiment and theory has not been achieved yet. The difficulty lies in the delicate nature of H-bond structure of water which is affected by not only the directional H-bond strength but also the non-directional van der Waals interaction, which is missing in conventional GGA functional. We performed ab initio molecular dynamics simulations based on the recently developed SCAN meta-GGA functional, which has been proved to be an excellent choice for liquid water (PNAS 114, 10846). We calculated the IR spectra of deuterated water (and ice Ih). Experimental features, such as peaks corresponding to stretching modes, bending mode and libration mode of water molecules, are much better reproduced by the SCAN functional compared with GGA-PBE functional. We further analyze the improvement of computed IR spectra based on the electronic structure improvement provided by SCAN. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P57.00006: A viscoelastic hydrodynamic theory of acoustic excitations in liquids Zhikun Cai, Yang Zhang Long wavelength longitudinal phonons can propagate in liquids, but whether transverse phonons exist in liquids has been long debated. The classic hydrodynamic theory refutes the existence of transverse phonons in liquids because the transverse current fluctuation is not directly coupled with the density fluctuation and the Brillouin zone is not well-defined. In this work, we generalize the hydrodynamic theory by introducing viscoelastic response and longitudinal-transverse coupling into the stress tensor in the hydrodynamic equations. As a result, the transverse acoustic excitation emerges not only in the current correlation functions but also in the dynamic structure factor, which directly characterizes the density fluctuation. From this derivation, we show the Ioffe-Regel phonon localization condition can be determined. This framework demonstrates another route to generalize the hydrodynamic theory, where the stress tensor plays a similar role as the memory function in the existing generalized hydrodynamics or memory-function approach. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P57.00007: All-atom Metadynamics Simulations of the Hierarchical Folding Dynamics of Proteins on the Timescale of Seconds Nathan Walter, Yang Zhang Molecular dynamics simulations have proven instrumental to the understanding of molecular biophysics. However, the temporal constraints of molecular dynamics simulations have limited attempts to capture the protein folding process at the atomic scale. Herein, we circumvent this limitation by using our all-atom metadynamics algorithm to directly sample the potential-energy landscape of numerous proteins. Previous applications of the original metadynamics method to proteins penalized select collective variables or dihedral angles assumed to be principle to the folding process. Rather, our method penalizes the full coordinate space of the protein resulting in 3N-dimensional sampled energy-landscape, unbiased by a priori assumptions. With our all-atom sampling, a single simulation captures the folding and unfolding process multiple times, enabling the simulation of protein dynamics on the timescale of seconds, orders of magnitude longer than recorded molecular dynamics simulations. From the sampled energy landscape, we predict the folded state, the activation barrier and the timescale associated with the folding processes of the proteins. Herein, we will present these findings for several well-studied proteins, to validate our results, and several new proteins, as a novel extension. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P57.00008: Overcoming the time limitation in Molecular Dynamics simulation of crystal nucleation: a persistent-embryo approach Yang Sun, Huajing Song, Feng Zhang, Lin Yang, Zhuo Ye, Mikhail Mendelev, Cai-Zhuang Wang, Kai-Ming Ho The homogeneous crystal nucleation from liquid in most cases is too rare to be accessed within the limited timescales of the conventional molecular dynamics (MD) simulation. In this talk, we present a “persistent embryo” method to facilitate crystal nucleation in MD simulations. We applied this method to the pure Ni for a moderate undercooling where no nucleation can be observed in the conventional MD, and obtained nucleation rate in good agreement with the experimental data. With MD simulation, we were able to describe the shape of the nucleus and derived the interfacial free energy as a function of the temperature. Moreover, the method is applied to simulate an even more sluggish event: the nucleation of the B2 phase in a strong glass-forming Cu-Zr alloy. The nucleation rate was found to be 8 orders of magnitude smaller than Ni at the same undercooling, which well explains the good glass formability of the alloy. This work opens a new way to study solidification under realistic experimental conditions via atomistic computer simulation. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P57.00009: Nanoconfinement and phase transitions of water in protein crystals David Moreau, Hakan Atakisi, Robert Thorne The structure, equilibrium states, and phase transitions of water is altered by nanoconfinement. Protein crystals provide a unique platform to study nanoconfined water as they can be grown large enough to perform macroscopic measurements, their electron densities can be reconstructed through x-ray diffraction, a large array of confinement geometries are available, and the confinement is truely biologically relevant. By monitoring the time dependence of crystal diffraction during and following cooling to temperatures between 180 and 260K, we have studied how and when ice forms and the properties of this ice. We have also performed high precision density measurements allowing us to estimate the density of solvent confined within the solvent cavities. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P57.00010: Kosmotrope and Chaotrope Salts Influence on Water Structural Relaxation Investigated by Coherent Quasielastic Neutron Scattering Antonio Faraone, Erkan Senses, Eugene Mamontov Aqueous solutions of salts have been the focus of many experiments and simulations because of their relevance in physics, chemistry, and biology. Typically, anions and cations are classified according to the Hofmeister series for their properties of enhancing or weakening the hydrogen bond network of water. However, at the molecular level the distinction between kosmotrope (structure makers) and chaotrope (structure breakers) is less clear as both have a coordination shell of water molecules around them and can perturb the water structure beyond the first hydration shell. Using coherent quasielastic neutron scattering, we have investigated the structural dynamics of NaCl/D2O and KCl/D2O, NaCl and KCl being well studied kosmotrope and chaotrope, respectively. By probing the dynamics of these systems at the structure factor peak, we measured how different salts affect the structural relaxation of water. Both salts clearly reduce the de Gennes narrowing effect observed in water. Interestingly, as the salt concentration is increased, the de Gennes narrowing effect gets slightly more pronounced in NaCl/D2O whereas it keeps getting weaker in KCl/D2O. These results indicate that competing effects play a role in determining how salts affect water structural relaxation. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P57.00011: Seeking Structural Clues to Propensity in Simulations of Glass Formers Cordell Donofrio, Eric Weeks Propensity in a supercooled liquid is the idea that at a given time, the structure surrounding each particle exerts some influence on the magnitude of the particle's subsequent motion. Particles with the tendency to move extreme distances are observed by running multiple simulations of the system; each with the same starting positions but with randomized velocities consistent with the temperature. Specifically, we are examining the Kob-Andersen binary Lennard-Jones glass former. We look to correlate the particles' propensity with various structural factors. Further confirmation of important structures is determined by changing the size of one particle in the simulation and re-doing the analysis. This should in principle reveal structures that play an important role in the glass transition. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P57.00012: Continuous Freezing and Melting of Water in Proximity to
Nanostructured CuO Coatings1 James Torres, Zachary Buck, Helmut Kaiser, Robert Winholtz, Haskell Taub, Madhusudan Tyagi, Kenneth Herwig, Eugene Mamontov, G. Ehlers, Flemming Hansen Nanostructured, hydrophilic CuO coatings have been used to enhance the performance of oscillating heat pipes. We use neutron scattering techniques to elucidate the effect of these coatings on the microscopic structure and dynamics of interfacial fluid (water). Our incoherent elastic neutron scattering and neutron diffraction measurements have shown that CuO interfacial water freezes continuously over the range 280 K to 200 K consistent with the formation of amorphous ice. To investigate the dynamics of water interacting with these CuO surfaces on a ns to ps time scale, we are using two backscattering spectrometers [HFBS at the NIST Center for Neutron Research and BASIS at the Oak Ridge Spallation Neutron Source (SNS)] and the cold neutron chopper spectrometer CNCS at the SNS. Preliminary measurements on the CNCS indicate that at 293 K the dominant contribution to the quasielastic spectra is from the diffusion of bulk-like water. However, spectra at 250 K taken on the HFBS from a sample of comparable hydration show water diffusing about 4 times slower than bulk supercooled water at this temperature. We expect our measurements on BASIS to yield a detailed temperature dependence of the diffusion rate of the interfacial water. |
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