Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K47: Bulk Water: Experiment and Theory |
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Sponsoring Units: DCP Chair: David Chandler, Sandia National Laboratory Room: 312 |
Wednesday, March 16, 2016 8:00AM - 8:12AM |
K47.00001: Accurate path integral molecular dynamics simulation of ab-initio water at near-zero added cost Daniel Elton, Michelle Fritz, José Soler, Marivi Fernandez-Serra It is now established that nuclear quantum motion plays an important role in determining water’s structure and dynamics. These effects are important to consider when evaluating DFT functionals and attempting to develop better ones for water. The standard way of treating nuclear quantum effects, path integral molecular dynamics (PIMD), multiplies the number of energy/force calculations by the number of beads, which is typically 32. Here we introduce a method whereby PIMD can be incorporated into a DFT molecular dynamics simulation at virtually zero cost. The method is based on the cluster (many body) expansion of the energy. We first subtract the DFT monomer energies, using a custom DFT-based monomer potential energy surface. The evolution of the PIMD beads is then performed using only the more-accurate Partridge-Schwenke monomer energy surface. The DFT calculations are done using the centroid positions. Various bead thermostats can be employed to speed up the sampling of the quantum ensemble. The method bears some resemblance to multiple timestep algorithms and other schemes used to speed up PIMD with classical force fields. We show that our method correctly captures some of key effects of nuclear quantum motion on both the structure and dynamics of water. [Preview Abstract] |
Wednesday, March 16, 2016 8:12AM - 8:24AM |
K47.00002: ABSTRACT WITHDRAWN |
Wednesday, March 16, 2016 8:24AM - 8:36AM |
K47.00003: X-ray absorption of liquid water studied by advanced \textit{ab initio} methods Zhaoru Sun, Jianping Wang, Wei Kang, Roberto Car, Xifan Wu Oxygen K-edge X-ray absorption spectra (XAS) provide a sensitive local probe of the H-bond structure of liquid water. Based on the static COHSEX approach, we computed the XAS spectra of liquid water from molecular structures generated by \textit{ab initio} molecular dynamics (AIMD) simulations using a van der Waals (vdW) inclusive hybrid functional (PBE0) that gives ambient water structure in quantitative agreement with experiment [JCP 141, 084502 (2014)]. We find that good agreement between experimental and theoretical XAS requires both improved molecular modeling and excitation treatment. In our simulation the over-structured H-bond network resulting from GGA-AIMD is systematically reduced as the directional H-bond strength is lowered by the mitigated self-interaction error in PBE0 and the increased population of interstitial water molecules promoted by vdW interactions. The better H-bond structure in turn gives improved XAS spectra. Moreover, we find that the orbitals obtained from the self-consistent diagonalization of the self-energy are crucial in obtaining spectra that compare well with experiment. [Preview Abstract] |
Wednesday, March 16, 2016 8:36AM - 8:48AM |
K47.00004: Why does hydronium diffuse faster than hydroxide in liquid water? Lixin Zheng, Biswajit Santra, Robert DiStasio, Michael Klein, Roberto Car, Xifan Wu Experiments show that the hydronium ion (H$_3$O$^+$) diffuses much faster than the hydroxide ion (OH$^-$) in liquid water. {\it ab initio} molecular dynamics (AIMD) simulations correctly associated the diffusion mechanism to proton transfer (PT) but have been unable so far to clearly identify the reason for the faster diffusion of hydronium compared to hydroxide, as the diffusion rate was found to depend sensitively on the adopted functional approximation. We carried out AIMD simulations of the solvated water ions using a van der Waals (vdW) inclusive PBE0 hybrid density functional. It is found that not only hydronium diffuses faster than hydroxide but also the absolute rates agree with experiment. The fast diffusion of H$_3$O$^+$ occurs via concerted PT that enables the ion to jump across several H-bonded molecules in successful transfer events; in contrast, such concerted motion is significantly hindered in OH$^-$ where the ion is easily trapped in a hyper-coordination configuration (a local solvation structure that forbids PT). As a result multiple PT events are rare and the diffusion of OH$^-$ is significantly slowed down. Such a clear difference between the two ions results from the combined effect of vdW interactions and self-interaction correction. [Preview Abstract] |
Wednesday, March 16, 2016 8:48AM - 9:00AM |
K47.00005: Electrical Mobility of Protons and Proton-Holes in Pure Water Characterized by Physics-Based Water Model. Binbin Jie, Chihtang Sah Pure water has been characterized empirically for nearly a century, as dissociation into hydronium (H$_{\mathrm{3}}$O)$^{\mathrm{1+}}$ and hydroxide (HO)$^{\mathrm{1-}}$ ions. Last March, we reported that the \textasciitilde 40 year experimental industrial standard of chemical equilibrium reaction constant, the ion product, can be accounted for by a statistical-physics-based concentration product of two electrical charge carriers, the positively charged protons, p$^{\mathrm{+}}$, and the negatively charged proton holes or prohols, p$^{\mathrm{-}}$, with a thermal activation energy or proton trapping well depth of E$_{\mathrm{p+/p-}} \quad =$ 576 meV, in the 0-100$^{\mathrm{O}}$C pure liquid water. We now report that the empirically fitted industrial standard experimental data (1985, 1987, 2005) of the two dc ion mobilities in liquid water, can also be accounted for by trapping-limited drift of protons and prohols through proton channels of lower proton electrical potential valleys, E$_{\mathrm{p+/0}} \quad \le $ E$_{\mathrm{p-/0}}$ \textless (E$_{\mathrm{p+/p\thinspace -}}$/3), in the tetrahedrally-directed electron-pair-bonded oxygen ions, O$^{\mathrm{2-}}$, in hexagonal lattice based on the 1935 Pauling statistical model using the 1933 Bernal-Fowler water rule. [Preview Abstract] |
Wednesday, March 16, 2016 9:00AM - 9:12AM |
K47.00006: Nuclear Quantum Effects in Different Ice Phases Marivi Fernandez-Serra, Betul Pamuk, Philip B. Allen We have previously explained that the anomalous isotope effect in hexagonal ices is liked to the anticorrelation between the covalent OH bond and the hydrogen bond by using the quasiharmonic approximation combined with {\it ab initio} density functional theory. [1] In this study, we show that similar physics plays a role in the isotope effect on temperature of the proton-order/disorder phase transition between ice XI and iceIh. By using a van der Waals density functional, we calculate a temperature difference between heavy and light ices of 6 K as compared to the experimental value of 4 K. [2] Furthermore, we extend our study to analyze the zero-point effects in different ice phases and ice-like structures with different densities and crystal structures to understand how this can be linked to the anomalous isotope effect in liquid water. [3] [1] B. Pamuk, {\it et al.} Phys. Rev. Lett. {\bf 108}, 193003 (2012). [2] B. Pamuk, P. B. Allen, M-V. Fernandez-Serra Phys. Rev. B {\bf 92}, 124105 (2015). [3] B. Pamuk, {\it et al.} in preparation. [Preview Abstract] |
Wednesday, March 16, 2016 9:12AM - 9:24AM |
K47.00007: Electron Traps at the Ice Surface Michel Bockstedte, Philipp Auburger, Anja Michl Water, water clusters and ice possess the fascinating ability to solvate electrons. On the surface of water cluster$^{1}$ and thin crystalline ice structures on a metal substrate$^{2}$ long-living solvated electron states were observed that evolve from pre-existing surface traps. The identification of such traps provides important insight into the electronic structure of the water or ice surface, and the dissociative interaction of electrons with adsorbates. Models$^{2,3}$ based on the bilayer terminated Ih-(0001) surface related such traps to orientational defects or vacancies. So far, the understanding of the electronic structure of the ice surface with the electron traps is incomplete. Here we address this issue including also water ad-structures$^{4}$ within hybrid density functional theory and many-body perturbation theory (G$_{0}$W$_{0}$). We identify a hierachy of traps with increasing vertical electron affinity, ranging from hexagon adrows to clusters of orientational defects and vacancies with dangling OH-groups.\\ [1] Siefermann and Abel, Angew. Chem. Int. Ed. 50, 5264 (2011).\\ [2] Bovensiepen et al., J. Chem. Phys. C 113, 979 (2013).\\ [3] Hermann et al., J. Phys.: cond. matter 20, 225003 (2008).\\ [4] Mehlhorn and Morgenstern, Phys. Rev. Lett. 99, 246101 (2007) [Preview Abstract] |
Wednesday, March 16, 2016 9:24AM - 9:36AM |
K47.00008: Quantum Water Ice Nic Shannon, Owen Benton, Olga Sikora There is now a growing body of evidence, from both simulation [1] and experiment [2], that the protons in common, hexagonal water ice are not merely disordered, but mobile, collectively tunnelling from one configuration to another. In this talk we revisit the theory of proton correlations in hexagonal water ice, showing how the disordered state selected by the ice rules changes, once collective quantum tunnelling is taken into account [3]. We find that correlations are governed by a lattice-gauge theory with exactly the same structure as electromagnetism, in which the low--energy excitations of protons have the character of ``photons''. The predictions of the quantum theory are shown to be in quantitative agreement with the results of quantum Monte Carlo simulations of hexagonal water ice, and to reproduce the ``wings'' of incoherent inelastic neutron scattering observed by Bove et al. [2]. These results raise the intriguing possibility that the protons in hexagonal water ice could form a quantum liquid with many of the same properties as the quantum spin liquids sought in frustrated magnets. [1] C. Drechsel-Grau and D. Marx, Phys. Rev. Lett. 112, 148302 (2014). [2] L. Bove et al., Phys. Rev. Lett. 103, 165901 (2009). [3] O. Benton, O. Sikora and N. Shannon, arXiv:1504.04158. [Preview Abstract] |
Wednesday, March 16, 2016 9:36AM - 9:48AM |
K47.00009: Role of Inter-molecular Charge Transfer in Simulating Concentration Dependent Water Diffusivity of Aqueous Salt Solutions Yi Yao, Max Berkowitz, Yosuke Kanai The translational diffusivity of water in solutions of alkali halide salts depends on the identity of ions, exhibiting dramatically different behavior even in solutions of similar salts of NaCl and KCl. The water diffusion coefficient decreases as the salt concentration increases in NaCl. Yet, in KCl solution it slightly increases and remains above bulk value as salt concentration increases. Previous classical molecular dynamics simulations have failed to describe this important behavior even with polarizable models. Here we show that the missing physical effect in previous simulations was charge transfer; its inclusion produces results in a quantitative agreement with experiments. We found that the concentration-dependent diffusivity reflects the importance of many-body effects among the water molecules when the ions are present. Explicit inclusion of charge transfer allows us to model accurately the difference in the concentration-dependent water diffusivity between Na$^{+}$ and K$^{+}$ ions in simulations, and it is likely to impact modeling of a wide range of systems. [Preview Abstract] |
Wednesday, March 16, 2016 9:48AM - 10:00AM |
K47.00010: Ab initio Determination of Formation Energies and Charge Transfer Levels of Charged Ions in Water. Anoop Kishore Vatti, Mira Todorova, Joerg Neugebauer The ability to describe the complex atomic and electronic structure of liquid water and hydrated ions on a microscopic level is a key requirement to understand and simulate electro-chemical and biological processes. Identifying theoretical concepts which enable us to achieve an accurate description in a computationally efficient way is thereby of central importance. Aiming to unravel the importance and influence of different contributions on the hydration energy of ions we perform extensive ab-initio molecular dynamics simulations for charged and neutral cations (Zn, Mg) and anions (Cl, Br, I) in water. The structural correlations and electronic properties of the studied ions are analysed and compared to experimental observations. Following an approach inspired by the defect chemistry in semiconductors [1] and aligning the water band edges on an absolute scale allows us to benchmark the calculated formation energies, identify transition states and compare the results to experiment. Based on these results we discuss the performance of various DFT xc-functionals to predict charge transfer levels and photo-emission experiments [2]. [1] M. Todorova and J. Neugebauer, Phys. Rev. Applied 1, 014001 (2014). [2] B. Winter et al., JACS 127, 7203 (2005). [Preview Abstract] |
Wednesday, March 16, 2016 10:00AM - 10:12AM |
K47.00011: Orientational relaxation of OH-bond in water over short to intermediate timescales Ping-Han Tang, Suan-Jen Lin, Ten-Ming Wu By simulating the rigid simple point charge extended model at 300$K$, the orientational relaxation of the OH-bond in water was investigated over short to intermediate timescales, within which molecules undergo inertial rotation and libration and then enter the rotational diffusion regime. Simulated molecules were classified into subensembles according to their H-bond connections. For global molecules and classified subensembles of molecules, the simulation results of the orientational time correlation functions (TCFs) were compared with the second-cumulant predictions obtained using the rotational stable instantaneous-normal-mode (INM) spectra and the power spectra of angular velocity autocorrelation functions (AVAFs). On short timescales, the OH-bond in water behaves similar to an inertial rotor and its anisotropy is lower than that of a water molecule. For molecules connected with three or more H-bonds, the OH-bond orientational TCFs are characterized by a recurrence, which is an indication for OH-bond libration. By contrast, the OH-bond orientational TCFs of molecules initially connected with one or two H-bonds decay monotonically or exhibit a weak recurrence, indicating rapid relaxation into the rotational diffusion regime after the initial Gaussian decay. [Preview Abstract] |
Wednesday, March 16, 2016 10:12AM - 10:24AM |
K47.00012: What is the effective molecular polarizability of water in condensed phases? Xiaochuan Ge, Deyu Lu Electronic polarization plays a crucial role in determining the structural and dynamical properties of water with different boundary conditions. Although it is well known that the molecular polarization in condensed phases behaves substantially differently from that in the vacuum due to the intermolecular interaction, these environmental effects have not been fully understood from first principles methods. As a result, how to rigorously define and calculate the effective molecular polarizability of a water molecule in different chemical environments remains an open question. The answer to this question not only improves our fundamental understanding of water, but also has immediate practical impact on computational modeling of water, e.g, through an accurate polarizable force field model. A main challenge to this puzzle arises from the intrinsic non-local nature of the electronic susceptibility.Recently we developed an ab initio local dielectric response theory [arxiv 1508.03563] that partitions dielectric response in real space based on a Wannier representation. In this work we apply this method to compute the effective molecular polarizability of water in the condensed phase, and discuss how the effective molecular polarizability evolves from gas phase to the condensed phase. [Preview Abstract] |
Wednesday, March 16, 2016 10:24AM - 10:36AM |
K47.00013: Metastable liquid-liquid phase separation and criticality in water-like models RAKESH SINGH, John Biddle, Pablo Debenedetti, Mikhail Anisimov Water shows intriguing thermodynamic and dynamic anomalies in the supercooled liquid state. A possible explanation of the origin of these anomalies lies in the existence of a metastable first order liquid-liquid phase transition (LLPT) between two (high and low density) forms of liquid water. Unambiguous experimental proof of existence of LLPT in bulk supercooled water is so far hampered by ultra-fast ice crystallization. Computer simulations of water models are therefore crucial for exploring the possibility of LLPT in deeply supercooled water. We present computer simulation results that elucidate the possibility of a metastable LLPT in one of the most accurate atomistic models of water, TIP4P/2005. To describe the computed properties, we have applied two-state thermodynamics, viewing water as a non-ideal mixture of two inter-convertible states. The thermodynamic behavior of the model in the one-phase region suggests the existence of energy-driven LLPT. We compare the behavior of TIP4P/2005 with other popular water models, and with real water, all of which are well-described by two-state thermodynamics. Additionally, we also elucidate the relation between the phenomenological order parameter of the two-state thermodynamics and the microscopic nature of the low-density structure. [Preview Abstract] |
Wednesday, March 16, 2016 10:36AM - 10:48AM |
K47.00014: Structure Evolution of Metal Nanoparticles in Water Environment. Yi Gao, Beien Zhu Metal nanoparticles have drawn extensive attentions in materials science due to their widespread applications in electronics, engineering and catalysis. A very fundamental question is their structure evolution and surface segregation. Many recent observations have shown that reactive gases or supports may have strong effects on the morphology change and surface segregation. However, the effect of water---the most common solvent and environment---has not received enough attention. Here, we will give two examples to show water adsorption could induce the morphology change and strong surface segregation tendencies for the metal nanoparticles. This finding not only prompts us to re-examine the potential effects of water on metal nanoparticles, but would be also very helpful as a guide for the further theoretical and experimental studies in this field. [Preview Abstract] |
Wednesday, March 16, 2016 10:48AM - 11:00AM |
K47.00015: Ionic structure in electrolyte confined by dielectric interfaces Yufei Jing, Vikram Jadhao, Jos W. Zwanikken, Monica Olvera de la Cruz The behavior of ions in liquids confined between macromolecules determines the outcome of many nanoscale assembly processes in synthetic and biological materials. To model these systems, both the macromolecules and the surrounding solvent are treated as continuous media characterized with different dielectric permittivities. As the macromolecule-liquid boundary is modeled as a dielectric interface, an important quantity of interest is the ionic structure in a liquid confined between two such interfaces. We compute the ionic structure in a model system of electrolyte confined by two planar dielectric interfaces using molecular dynamics simulations and liquid state theory. We give a comprehensive description of the effects of high electrolyte concentrations, multivalent ions, dielectric contrasts, and external electric field on the ionic distributions. We observe novel features in ionic structure near polarizable/unpolarizable macromolecules which is attributed to the competition between electrostatic and steric (entropic) interactions. We argue that the combined effect of ionic correlations and inhomogeneous dielectric permittivity significantly changes the character of the effective interaction between the two macromolecules. [Preview Abstract] |
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