Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K10: Focus Session: Frontiers in Computational Chemical Physics II |
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Sponsoring Units: DCP Chair: Rigoberto Hernandez, Georgia Tech Room: Baltimore Convention Center 302 |
Tuesday, March 14, 2006 2:30PM - 3:06PM |
K10.00001: First-principles statistical mechanics for heterogeneous catalysis Invited Speaker: We present a first-principles approach to heterogeneous catalysis that quantitatively describes the activity over a wide range of realistic environmental situations of varying temperatures and pressures. Within a first-principles statistical mechanics setup [1], density-functional theory is first used together with transition state theory to accurately obtain the energetics of all relevant processes. Subsequently the statistical mechanics problem is solved by kinetic Monte Carlo simulations. This two-step approach enables us to gain microscopic insight into the system, following its full dynamics from picoseconds up to seconds and explicitly considering the detailed statistical interplay of all elementary processes, i.e., by fully accounting for the correlations, fluctuations and spatial distributions of the chemicals at the catalyst surface. In the application to CO oxidation at a RuO$_2$(110) model catalyst, we compute the composition and structure of the catalyst surface in reactive environments ranging from ultra- high vacuum to technologically relevant conditions with pressures of the order of atmospheres and elevated temperatures [2]. For all these conditions the obtained conversion rates are in unprecedented quantitative agreement with existing experimental data. The catalytic activity is narrowly peaked in environments, where the surface kinetics builds a disordered and dynamic adsorbate composition at the surface. In the full concert of the large number of processes occurring in this active state, the chemical reaction with the most favorable energy barrier contributes only little to the overall CO$_2$ production. \newline \newline [1] K. Reuter, D. Frenkel, and M. Scheffler, Phys. Rev. Lett. 93, 116105 (2004). \newline [2] K. Reuter and M. Scheffler, Phys. Rev. B, submitted. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K10.00002: Kinetics and equilibrium properties of supercooled salt solutions in contact with ice and water vapor Marcelo Carignano, Paul Shepson, Igal Szleifer The kinetics of ice growth from salt solutions and the final equilibium systems are studied by Molecular Dynamics simulations. Two systems are investigated. In one a liquid layer of supercooled salt solution is sourronded by ice walls. In the other, the liquid layer is in contact with an ice block on one side and water vapor on the other side. We found that the kinetics of ice growth in both cases is slower than the corresponding system with no ions. A faster ice growth is obserbed on the prismatic plane than on the basal plane of the ice. In the cases where complete freezing is achieved, the final systems have all the ions participating in cluster formations. In the system with water/vapor interface, complete freezing is not reached, leaving the ions solvated in a quasi liquid layer. The density profiles of the ions show their preference to be closer to the water/ice interface than to the water/vapor interface. The simulations enable the understanding of the mechanisms for ice formation under different solution conditions, and the final equilibrium properties of the studied systems. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K10.00003: Thermodynamic and Kinetic differences between solid superheating and liquid supercooling Xian-Ming Bai, Mo Li When surface melting is properly suppressed, a solid can be heated above its melting temperature, which is called superheating. Although many explanations were proposed for solid superheating in the past century, its nature and extent still remain veiled. It is commonly assumed that solid superheating is a reverse process of liquid supercooling. The classical nucleation theory was directly applied to estimate the liquid nucleation rate and the upper limit of superheating despite the lack of decisive test of this analogy. Here we present our investigation of the thermodynamic and kinetic differences between superheating and supercooling via molecular-dynamics simulations. It is shown that the large elastic energy associated with internal melting and solid-liquid interface disorder play important roles in superheating. The growth rate is anisotropic for supercooling, but isotropic for superheating. Supercooling can be well described by the classical nucleation theory, whereas superheating shows many exceptions. The underlying mechanisms for these differences are discussed. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K10.00004: Why does 5-mthyl-2-pyrimidinone fluoresce? An ab initio study of the photophysics of a fluorescent DNA pyrimidine analog. Kurt Kistler, Spiridoula Matsika The photophysical properties of gas phase 5-methyl-2-pyrimidinone, a fluorescent DNA/RNA pyrimidine analog, have been calculated using multi-reference configuration-interaction with three levels of dynamical electron correlation included. The bright state at vertical excitation is S$_{2}$ ($\pi \pi $*) at 4.42 eV. An S$_{1}$-S$_{2}$ seam originates close to vertical, and a gradient pathway from this seam leads to a global minimum on the S$_{1}$ surface. A conical intersection between S$_{1}$ and S$_{0}$ (\textit{ci}01) was found, but all levels of theory show that this conical intersection is significantly higher than the minimum, supporting fluorescence from S$_{1}$. The details for this energetic inaccessibility, and the emission energy calculated (2.36 - 2.75 eV), depend on the level of theory used. Pathways along the S$_{1}$ surface when no dynamical or intermediate dynamical correlation is included show a small barrier along with a second minimum in the S$_{1}$ path from vertical to \textit{ci}01. When higher level of correlation is included ($\sim $130,000,000 configurations) the barrier vanishes, the secondary minimum becomes global, the energy of \textit{ci}01 increases, and the fluorescence energy becomes closer to experimental. Thus the importance of dynamical correlation when studying the photophysics of the DNA/RNA bases theoretically is underscored. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K10.00005: Interactions between nanoparticles in supercritical fluids Sergei Egorov This talk presents a theoretical study of interactions between sterically stabilized spherical nanoparticles in a supercritical solvent. The theory is used to analyze the effect of particle size, solvent density, and solvent-ligand interaction strength on the potential of mean force between the particles. Experimentally observed size- selective precipitation of nanoparticles is rationalized in terms of the behavior of the density profiles of stabilizing ligands as a function of particle size and solvent thermodynamic conditions. The theory yields the same general trends as observed in experiments, namely, an increased stability of nanoparticle dispersions at higher solvent densities and for smaller particle sizes. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K10.00006: Effects of co-adsorbed C and K on energetics of CO oxidation on Pd(111) Faisal Mehmood, Sergey Stolbov, Talat S. Rahman In the course of catalytic oxidation of CO, carbon may atomically adsorb on catalyst surface and change (presumably poison) its reactivity. On the other hand, the reactivity of many catalysts is enhanced upon alkali co-adsorption. To gain insight into the nature of these effects, we carried out ab initio electronic structure calculations based on density functional theory with the generalized gradient approximation. The energetics and reaction pathways for CO oxidation on the clean Pd(111) and on Pd(111) co-adsorbed with C or K are calculated. We find that even at coverages as small as 1/12 ML, co-adsorbed C increases the activation energy barrier for the reaction by 20{\%}, while K reduces it by 10{\%}. This clearly shows the role of K as a promoter and C as a poison for this catalytic reaction. To understand the microscopic mechanism of these phenomena, we analyze the effects of the co-adsorbate coverage and the CO --- co-adsorbate distance on the activation energy barriers and the local densities of electronic states and valence charge densities calculated for the initial and transition states of the reaction. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K10.00007: Parallel Temperature-Accelerated Dynamics Simulations of Epitaxial Growth Y. Shim, J. G. Amar, B. P. Uberuaga, A. F. Voter The temperature-accelerated dynamics (TAD) method is a powerful tool for carrying out non-equilibrium simulations of systems with infrequent events over extended timescales. However, since the computational time for a typical TAD simulation increases rapidly with the number of atoms $N$, TAD simulations have so far been limited to relatively small system sizes. By applying a recently proposed synchronous sublattice algorithm to parallel TAD simulations, we have been able to simulate the evolution of systems over much larger length- as well as time-scales. As a first test of our method, we have carried out simulations of the surface diffusion of Cu atoms on the Cu(100) surface. In contrast to serial TAD simulations for which the computational time scales as $N^{2.5} - N^3$, in our parallel TAD simulations the computational time scales as $log(N)$ and may even be independent of $N$ for larger system sizes. In particular we find that for intermediate size systems our parallel TAD simulations are several orders of magnitude faster than the corresponding serial TAD simulations. Preliminary results for low-temperature multilayer Cu/Cu(100) growth obtained using parallel TAD simulations are also presented. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K10.00008: Hydration and Dewetting near Fluorinated Superhydrophobic Plates Ruhong Zhou, Xin Li, Jingyuan Li, Maria Eleftheriou The water dynamics near a nanoscale fluorinated (CF$_3$(CF$_{2}$) $_{7}$(CH$_{2}$)$_{2}$SiH$_{3}$) surface as well as the possible dewetting (water drying) transition within two such superhydrophobic surfaces (plates) have been studied with molecular dynamics simulations. A partial water dewetting with an expansion of approximately 7-8\AA~ and a water density $\sim$20\% lower than the bulk is found near the single superhydrophobic surface. More remarkably, a strong dewetting transition is found in the inter-plate region for the double plates with a critical distance $D_c$ up to 10\AA~ (3-4 water diameters). This transition, although occurring on a microscopic length scale, is reminiscent of a first order phase transition from liquid to vapor. Furthermore, simulation results show that the fluorinated carbons are more hydrophobic than their hydrogenated counterparts (with $D_c$ = 8.0\AA) in terms of the dewetting transition critical distance, despite their much larger partial charges and dipoles. The unusual superhydrophobicity of fluorocarbons is found to be related to their larger surface areas, while the intrinsic hydrophobicity is roughly the same for both fluorocarbons and hydrocarbons based on a detailed water-plate interaction energy profiling. Somewhat surprisingly, we find that even though the electrostatic energies do contribute slightly more in the fluorocarbon plates than the hydrocarbon plates, the van der Waals energies dominate the water-plate interactions (with more than 90\% contributions for close shells) and they contribute almost the same in both plates. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K10.00009: Metastable States and Population Dynamics in the Linkage Isomer Compound Sodium-nitro-prusside Bernard Delley, J\"urg Schefer, Dominik Schaniel, Theo Woike Two long-lived metastable states S1 and S2 can be produced in Na$_2$[Fe(CN)$_5$NO]2H$_2$O by illumination with light in the spectral range 380-580 nm. Local minima in the ground state energy hypersurface as given by density functional theory are in agreement with the linkage isomerism model. We identify S2 with a side on bonded NO configuration and S1 with an inverted one. We have shown that that thermal properties, vibrational spectrum, optical properties and the Mossbauer spectrum are predicted in agreement with experiment. We find that the crystal model gives a significantly improved account of spectral differences between the ground and S1 state as compared to the free anion model. On the basis of the energy surfaces we estimate thermal depopulation rates and explore the dynamics of direct population of S1 induced by blue light. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K10.00010: Effect of the substrate-adsorbate coupling on the dispersion of phonons of CO on Cu(001). Talat S. Rahman, Klaus Peter Bohnen, Marisol Alcantara Ortigoza In order to understand the effect of the substrate on the dynamics of the adsorbate we have carried out first principle calculations of the dispersion of phonons of the c(2x2) CO overlayer on Cu(001) based on the Density Functional Theory, using ultrasoft pseudopotentials and the PBE-GGA approximation for the exchange-correlation energy. Comparing with frozen-substrate calculation, at the gamma point, we found that the frequencies of the C-O stretch mode, $\nu $1, and the adsorbate-substrate stretch mode, $\nu $2, are almost independent of the substrate. However, while $\nu $2 is dispersionless, $\nu $1 presents a 10 meV dispersion at the M-point. The frustrated translation and frustrated rotation modes present a dispersion of less than 5 meV, nevertheless having the frustrated translation mode such a low energy a $\sim $1 meV dispersion is rather considerable. Moreover, Helium atom scattering studies of this mode found that it splits into two branches everywhere other than the $\Gamma $ point; they attributed one branch to the frustrated translation mode of a perfect c(2x2) CO overlayer, and the other one to defects in the same. Here we show that the splitting is an intrinsic feature of the perfect c(2x2) CO overlayer. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K10.00011: Theory of Electronic Structure and Nuclear Quadrupole Interactions in the BF3-NH3 Complex R.H. Pink, Dip N. Mahato, M.B. Huang, T.P. Das, Archana Dubey, Lee Chow, Mahendra K. Mahanti, R.H. Scheicher Boron Trifluoride is widely used as a catalyst in chemical research and industry forming complexes in the process with other molecules like NH$_{3}$, H$_{2}$O, and CH$_{3}$OH. We have studied the BF$_{3}$-NH$_{3}$ molecular complex using Hartree-Fock procedure combined with many-body perturbation theory. The B-N bond is found to involve about 75{\%} covalent and 25{\%} van der Waals characters. The calculated geometry of the complex compares well with experimental results. The nuclear quadrupole interaction parameters for the $^{19}$F* nucleus [1] show good agreement with experiment for both solid BF$_{3}$ and the complex including the nuclear quadrupole coupling constants and the sizable asymmetry parameters. The $^{11}$B quadrupole coupling constant is however found to be about 1.5 times the available experimental value [2] for solid BF$_{3}$, possibly due to the influence of intermolecular bonding. \newline \newline [1] K. Bertholdt et al. J. Mol. Struct. 192, 199 (1989) \newline [2] P.A. Casabella and T. Oja J. Chem. Phys. 50, 4814 (1969) [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K10.00012: Theory of Nuclear Quadrupole Interaction in Trifluoro-Aminoboranes [BF$_{3}$-NH$_{3-X}$(CH$_{3})_{X}$] Mahendra K. Mahanti, Archana Dubey, H.P. Saha, Lee Chow, R.H. Scheicher, R.H. Pink, Dip N. Mahato, M.B. Huang, T.P. Das The understanding of the catalytic properties of BF$_{3}$ of great current interest require information about the electronic structures of the associated complexes of BF$_{3}$ with the molecules involved in the process. In this work we have studied the corresponding complexes for methylamines using the Hartree-Fock procedure combined with many-body perturbation theory. Results will be presented for the natures of the three complexes and the $^{19}$F* quadrupole coupling constants and asymmetry parameters for which experimental data [1] are available. Comparisons will be made with the corresponding properties for BF$_{3}$-NH$_{3}$, allowing valuable insights into the trends in the electron distributions in this family of complexes. \newline \newline [1] K. Bertholdt et al., J. Mol. Struct. 192, 199 (1989) [Preview Abstract] |
Tuesday, March 14, 2006 5:18PM - 5:30PM |
K10.00013: Electron Distribution in Solid BF$_{3}$ and BF$_{3}$ -- NH$_{3}$ -- Associated Nuclear Quadrupole Interactions Dip N. Mahato, R.H. Pink, M.B. Huang, T.P. Das, Archana Dubey, Lee Chow, Mahendra K. Mahanti, R.H. Scheicher The electronic structures of BF$_{3}$ and the complex BF$_{3}$ -- NH$_{3 }$in the solid state[1] have been studied by the Hartree-Fock Cluster Procedure and including many-body effects by perturbation theory. The motivation is to understand the nature of the bonding between the individual molecules in the solid and changes in the bonding within the molecules. We also have attempted to understand the differences in the nuclear quadrupole interactions of $^{11}$B and $^{19}$F* nuclei in the free molecules and the solid state systems and have compared the results of theory with available experimental results[2] in solid state. Nuclear quadrupole interaction results for $^{14}$N and deuteron will also be presented in the complex. \newline \newline [1] D. Mootz and M. Steffen, Angew. Chem. Int. Ed. Engl. 19, 483(1980). \newline [2] P.A. Casabella and T. Oja, J. Chem. Phys. 50, 4814(1969); K. Berthholdt et al, J. Mol. Struct. 192, 199(1989). [Preview Abstract] |
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