Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J35: Energy Landscapes in Clusters, Materials, and Biology IV |
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Sponsoring Units: DCP Chair: A. Heuer, University of Muenster Room: LACC 511B |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J35.00001: What Must We Know to Gain Useful Knowledge from a Complex Surface? Invited Speaker: Complex potential surfaces can be analyzed, in principle, to yield virtually any desired information about their topographies. However for a system of more that about 20 particles, the surfaces are so complex that the only feasible ways to work with them involve the extraction of some minimal set of data that can provide the information we most want. Molecular dynamics and Monte Carlo searches, especially with long-range exploration algorithms or coarse-graining, are two well-studied approaches. Kinetic methods based on master equations provide another with the power to reach long time scales. Their largest difficulty is finding a way to construct a master equation based on a suitably small subset of the information that will yield, with reliability, the important rates we need, specifically the rates of the slow but attainable processes. We review several approaches that we have been pursuing: reduction of the kinetics to interbasin passage, reduction of the number of coordinates to just the most important, by principal component analysis, and sampling of pathways on the surface. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:27PM |
J35.00002: The potential energy landscape of glass-forming materials: a computer study Invited Speaker: Glass-forming systems show remarkable properties in the supercooled liquid phase (e.g. non-Arrhenius temperature dependence or strong connections between thermodynamics and dynamics) as well as in the glassy phase (e.g. the emergence of tunneling centres at very low temperatures). Another aspect deals with the surprisingly fast dynamics of ions in some glassy materials. In this talk a concept is presented which allows one to describe all these features in a coherent way. It is based on the study of the potential energy landscape. Using appropriate numerical tools as well as physical insight it is possible to elucidate the underlying nature of the many relevant features in the supercooled as well as in the glassy regime. Among other things the difference between strong glasses and fragile glasses (Arrhenius vs. Non-Arrhenius temperature dependence) can be expressed in terms of specific properties of the potential energy landscape. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J35.00003: The role of electrosn in the structure of liquid water Maria Victoria Fernandez-Serra, Emilio Artacho The structure of liquid water is analyzed by using DFT-based Ab initio molecular dynamic simulations. The essential role that the electronic degrees of freedom play in the the hydrogen bond (HB) interaction is described and a new HB definition based in electrons and not in geometric parameters is proposed. Using this HB probe the structure and dynamics of the HB network in liquid water will be presented, providing new insights interestingly different from the picture that emerges from simulations based on geometrical criteria. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J35.00004: Air-liquid interface of ionic liquid-water binary system studied by surface tension measurement and sum-frequency generation spectroscopy Jaeho Sung, Yoonnam Jeon, Doseok Kim, Takashi Iwahashi, Toshifumi Iimori, Kazuhiko Seki, Yukio Ouchi Surface of room-temperature ionic liquid (RIL)+water mixture is investigated using surface tension measurement and surface sum- frequency generation (SFG) vibrational spectroscopy. Results indicate the liquid surface is mostly covered by the cations at a very low bulk concentration (less than 0.02 bulk mole fraction). Increase of surface tension from 0.016 up to $\sim$0.05 mole fraction suggested that the anions start to appear at the surface from $\sim$0.016 mole fraction until the anions and cations are equally populated at c$\sim$0.05 or higher. From the analysis of the SFG spectra, the terminal CH$_3$ group of the butyl chain is polar-oriented with its symmetry axis aligning rather vertical to the surface for the whole range of concentration. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J35.00005: Water condensation in proximity of a nanoscale asperity: A density functional description for isotropic fluid Pavel Paramonov, Sergei Lyuksyutov A quantitative understanding of water condensation phenomena in proximity of nano- asperities under ambient humidity is important for different applications from scanning probe microscopy to macroscopic adhesion and friction. A non-local density functional formalism is used to describe an equilibrium distribution of the water-like fluid in the asymmetric nano-scale junction. The model system contains spherically curved and planar surfaces presenting an atomic force microscope (AFM) tip dwelling above the surface. The hydrogen bonding dominated in intermolecular attraction of the fluid is modeled as a square well potential with two adjustable energy and length parameters characterizing well's depth and width. The size and shape of the liquid meniscus formed between the surfaces with a given affinity to the fluid are determined for the different values of the ambient humidity. This model can be easily generalized for more complex geometries and effective intermolecular potentials. The results of our study establish a basic framework for the density functional description of the system with orientational anisotropy in the fluid induced by the non-uniform external electric field. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J35.00006: Dynamics of Water in AOT Reverse Micelles Probed Using Ultrafast IR Vibrational Echo and Pump-Probe Spectroscopies Ivan Piletic, Howe Siang Tan, Michael Fayer Water is used extensively as a solvent in chemistry and is ubiquitous in biological systems. Water's unique properties are intimately related to its dynamic hydrogen bond network. In addition to the bulk, water is often found in nanoscopic environments. Therefore, it is important to understand the dynamics of water that is nanoscopically confined and to compare it with bulk water dynamics. Nanoscopic pools of water (1.7 -- 4.0 nm diameter) in Aerosol-OT reverse micelles were directly probed using ultrafast vibrational echo and pump-probe spectroscopies on the OD hydroxyl stretch mode of water (5{\%} HOD in H$_{2}$O). The data are compared with experiments conducted on bulk water as well as 6M NaCl solution. Fits of the vibrational echo data demonstrate that the dynamics of water slow down substantially with decreasing reverse micelle size. The fastest dynamics ($\sim $50 fs) which reflect local hydrogen bond fluctuations, are similar to bulk water. The longer time scale dynamics are attributed to the dissociation and reformation of hydrogen bonds, and slow significantly ($\sim $10 times) as the nanopool size is reduced. Lifetime and anisotropy measurements using pump-probe spectroscopy also display similar size dependences. The vibrational echo and pump-probe data clearly distinguish the dynamics of bulk water and concentrated NaCl solutions from the dynamics of nanoscopic water confined in reverse micelles. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J35.00007: Nuclear Quantum Effects on Molecular Packing in Light and Heavy Water Matthew Glover, Matt Probert Light water shows a temperature of maximum density (TMD) at 4$^{\circ}$C, below which the liquid begins to expand upon cooling. Hydrogen bonding is thought to be responsible for the anomalous density behaviour below the TMD: molecules in the liquid must move apart in order to form hydrogen bonds which acts to drive down the density. The density maximum is thought to mark the point during cooling at which formation of an open hydrogen-bonded tetrahedral network becomes energetically preferred over contraction familiar in simple liquids. In heavy water, the TMD leaps upwards to 11$^{\circ}$C. The lower TMD in light water might be supposed to be a result of breakdown in the open hydrogen-bonded network due to NQD: orientational delocalisation of molecules weakens hydrogen bonding and allows a higher density at a lower temperature. We have analysed first- and second-nearest-neighbour distances in relation to molecular orientation using results from path integral molecular dynamics simulation. A simple breakdown in tetrahedral order upon density increase due to orientational delocalisation is not found in light water. Rather, NQD in light water permits hydrogen bonds to become \emph{straighter} at higher density. The result is to increase the size of cavities in the tetrahedral network and thus provide accommodation for a greater number of interstitials which find homes in those cavities. A qualitative explanation for the phenomena is given in terms of the energetics of the water dimer. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J35.00008: Molecular dynamics behind the principle relaxation of water: Observation of inertia and memory effects Koji Yokoyama, A. Jason McNary, H. W. K. Tom, Eric Schwegler, Giulia Galli We report an analysis of the single dipole to collective dipole moment time correlation function (TCF) calculated from the molecular dynamics simulations of liquid water at room temperature with a rigid and nonpolarizable TIP5P water model [1]. The single to collective dipole moment TCF is especially important because it allows us to investigate the local relaxation processes in a cluster of molecules and it can be directly related to the dielectric constant via the Fatuzzo-Mason equation. We have calculated the collective dipole moment associated with the first through fourth solvation shells and observed retarded relaxation processes for outer shells. We have also discovered a long-term and long-range rephasing process of about 40 ps involving more than 500 molecules. [1] M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys. 112, 8910 (2000). [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J35.00009: Direct Observation of Heterogeneous Translational Motion at Tg Stephen Swallen, Mark Ediger Recent experiments have provided direct evidence of heterogeneous translational motion in a supercooled small molecule organic liquid, tris-naphthylbenzene. The early stages of diffusion were measured using neutron reflectivity, and indicate that translational motion near the glass transition temperature T$_{g}$ is qualitatively different than diffusion in ``normal'' liquids. The diffusion coefficient was found to be wave-vector dependent, D(q) $\propto $ q$^{-2}$, with a crossover to a q-independent value, D(q $\to $ 0), at a length scale of $\sim $22 nm at T$_{g}$. These results demonstrate that translational motion on the nanometer length scale can be extremely heterogeneous in a single component system near T$_{g}$, giving rise to large jumps of roughly 20 molecular diameters. This observation explains the unusually fast diffusion coefficients found in many materials near T$_{g}$, and also the unusually rapid crystallization of supercooled liquids. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J35.00010: Study of Short-Range Order in Supercooled Liquid Silicon by Beam-Line Electrostatic Levitation (BESL) T.H. Kim, G.W. Lee, A.K. Gangopadhyay, K.F. Kelton, B. Sieve, A.I. Goldman, T.J. Rathz, J.R. Rogers, R.C. Bradshaw, R.W. Hyers Previous studies of the liquid structure of supercooled Si by x- ray diffraction using electromagnetic and aerodynamic levitation have produced conflicting results. We describe a BESL technique that obtains complete diffraction patterns in 0.1 s using high- energy synchrotron x-rays, allowing the evolving structures of supercooled liquids to be measured continuously. Contrary to some molecular dynamic simulation studies, no first order liquid- liquid phase transition was observed in supercooled liquid Si over the measured temperature range (1100 $^{\circ}$C to 1600 $^{\circ}$C). The coordination number remained constant, in conflict with earlier measurements. Modeling suggests that the A5 structure of liquid Si distorts continuously toward cubic diamond structure with decreasing temperature. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J35.00011: Computer Simulations, Nucleation Rate Predictions and Scaling Barbara Hale Computer simulations which generate small molecular cluster size distributions for use in steady state nucleation rate predictions and direct molecular dynamics simulations of the nucleation process depend crucially on the properties of the effective pair potentials and on the modeled system's metastable conditions. The latter conditions often differ significantly from those under which the experimental data are taken. For argon, in particular, simulations using both truncated and full Lennard-Jones potentials have generated a range of results for the nucleation rate, J, which, in magnitude, appear to be inconsistent with the limited experimental rate data. We propose that log J should be plotted \textit{versus} a ``universal'' function of the scaled supersaturation, lnS/[T$_{c }$/T -- 1]$^{3/2}$, which incorporates scaling of metastable system conditions with potential model (or experimental) properties, as appropriate. This plot, which provides a more realistic comparison of predicted and experimental rates, is applied to water and to argon data. For argon, this plot emphasizes the limited range of simulation results at large scaled supersaturations and suggests that none of these predicted rates are inconsistent with the much smaller experimental nucleation rates taken at reduced scaled supersaturations. For water, the plot indicates that experimental nucleation rates are likewise well characterized by the scaled supersaturation function. [Preview Abstract] |
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J35.00012: Heat Capacities and Magic Numbers in Metal Clusters Michael Springborg, Valeri G. Grigoryan, Denitsa Alamanova Using our own {\it Aufbau/Abbau} method in performing umbiased structure optimization of isolated M$_N$ clusters (with M being a metal) together with the {\it Embedded-Atom} method for the calculation of the total energy of a given structure, we have optimized the structure of Ni$_N$, Cu$_N$, and Au$_N$ clusters with $N$ up to 150. By analysing the total energy as a function of $N$ particularly stable clusters, corresponding to the so-called magic numbers, are identified. Subsequently, we use the harmonic approximation in calculating the vibrational frequencies for each value of $N$. These are finally using in calculating the molar vibrational heat capacity $\bar C_V$ as a function of temperature $T$. It is been discussed whether, for a given $T$, $\bar C_V$ shows a dependence on $N$ that correlates with the occurrence of magic numbers. An explanation for the observations is being offered. [Preview Abstract] |
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