Title: Generation of palladium clusters on Au(111) electrodes: Experiments and simulations
Author(s): Del Popolo M., Leiva E., Kleine H., Meier J., Stimming U., Mariscal M., Schmickler W.
Applied Physics Letters, 81, No. 14, pp. 2635-2637 (SEP 30 2002)
The properties of palladium clusters, generated with the electrochemical scanning tunneling microscope, have been investigated both by experiments and by computer simulations. The clusters are found to be larger and more stable if the tip is moved further towards the electrode surface in the generation process. The simulations suggest that the larger clusters consist of a palladium-gold mixture, which is more stable than pure palladium. Dissolution of the clusters occurs from the edges rather than layer by layer. (C) 2002 American Institute of Physics.
Title: Equilibrium polymerization of cyclic carbonate oligomers. III. Chain branching and the gel transition
Author(s): Ballone P., Jones R.O.
Journal Of Chemical Physics, 117, No. 14, pp. 6841-6851 (OCT 8 2002)
Ring-opening polymerization of cyclic polycarbonate oligomers, where monofunctional active sites act on difunctional monomers to produce an equilibrium distribution of rings and chains, leads to a "living polymer." Monte Carlo simulations [two-dimensional (2D) and three-dimensional (3D)] of the effects of single [J. Chem. Phys. 115, 3895 (2001)] and multiple active sites [J. Chem. Phys. 116, 7724 (2002)] are extended here to trifunctional active sites that lead to branching. Low concentrations of trifunctional particles c(3) reduce the degree of polymerization significantly in 2D, and higher concentrations (up to 32%) lead to further large changes in the phase diagram. Gel formation is observed at high total density and sizable c(3) as a continuous transition similar to percolation. Polymer and gel are much more stable in 3D than in 2D, and both the total density and the value of c(3) required to produce high molecular weight aggregates are reduced significantly. The degree of polymerization in high-density 3D systems is increased by the addition of trifunctional monomers and reduced slightly at low densities and low c(3). The presence of branching makes equilibrium states more sensitive (in 2D and 3D) to changes in temperature T. The stabilities of polymer and gel are enhanced by increasing T, and-for sufficiently high values of c(3)-there is a reversible polymer-gel transformation at a density-dependent floor temperature. (C) 2002 American Institute of Physics.
Title: Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals
Author(s): Koval S., Kohanoff J., Migoni R.L., Tosatti E.
Physical Review Letters, 89, No. 18, Art. No. 187602 (OCT 28 2002)
Based on an accurate first principles description of the energetics in H-bonded potassium-dihydrogen-phosphate crystals, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Tunneling is allowed only for clusters involving correlated protons and heavy ion displacements, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its proton-mediated covalent bonding. The ensuing lattice expansion couples self-consistently with the proton off-centering, thus explaining both the giant isotope effect and its close connection with geometrical effects.
Title: Chemical potentials of water and organic solutes in imidazolium ionic liquids: a simulation study
Author(s): Lynden-Bell R.M., Atamas N.A., Vasilyuk A., Hanke C.G.
Molecular Physics, 100, No. 20, pp. 3225-3229 (OCT 2002)
The excess chemical potentials of a series of molecules dissolved in the ionic liquid dimethyl imidazolium chloride have been determined by a series of transformations. The molecules are water, methanol, dimethyl ether, acetone and propane. Water and methanol have large negative values of the excess chemical potential (-29 kJ mol-(1) and -14 kJ mol(-1) respectively); the polar molecules dimethyl ether and acetone have positive values of about 7 kJ mol(-1) while the value for propane is +26 kJ mol(-1). Hydrogen bonding to the anion plays an important part in the stabilization of water and methanol.
Title: Kinetic Monte Carlo study of electrochemical growth in a heteroepitaxial system
Author(s): Gimenez M.C., Del Popolo M.G., Leiva E.P.M.
Langmuir, 18, No. 23, pp. 9087-9094 (NOV 12 2002)
Structural and kinetic aspects of 2-D irreversible metal deposition under potentiostatic conditions are analyzed by means of dynamic Monte Carlo simulations employing embedded atom potentials for a model system. Three limiting models, all considering adatom diffusion, were employed to describe adatom deposition. The first model (A) considers adatom deposition on any free substrate site on the surface at the same rate. The second model (B) considers adatom deposition only on substrate sites which exhibit no neighboring sites occupied by adatoms. The third model (C) allows deposition at higher rates on sites presenting neighboring sites occupied by adatoms. Under the proper conditions, the coverage (theta) versus time (t) relationship for the three cases can be heuristically fitted to the functional form theta = 1 - exp(-betat(alpha)), where alpha and beta are parameters. We suggest that the value of the parameter alpha can be employed to distinguish experimentally between the three cases. While model A trivially delivers a = 1, models B and C are characterized by alpha < 1 and alpha > 1, respectively.
Title: Equilibrium polymerization of cyclic carbonate oligomers. II. Role of multiple active sites
Author(s): Ballone P., Jones R.O.
Journal Of Chemical Physics, 116, No. 17, pp. 7724-7732 (MAY 1 2002)
Ring opening polymerization of bisphenol A polycarbonate is studied by Monte Carlo simulations of a model comprising a fixed number of Lennard-Jones particles and harmonic bonds [J. Chem. Phys. 115, 3895 (2001)]. Bond interchanges produced by a low concentration (0.10%less than or equal toc(a)less than or equal to0.36%) of chemically active particles lead to equilibrium polymerization. There is a continuous transition in both 2D and 3D from unpolymerized cyclic oligomers at low density to a system of linear chains at high density, and the polymeric phase is much more stable in three dimensions than in two. The steepness of the polymerization transition increases rapidly as c(a) decreases, suggesting that it is discontinuous in the limit c(a)-->0. The transition is entropy driven, since the average potential energy increases systematically upon polymerization, and there is a steady decline in the degree of polymerization as the temperature is lowered. The mass distribution functions for open chains and for rings are unimodal, with exponentially decaying tails that can be fitted by Zimm-Schulz functions and simpler exponential forms. (C) 2002 American Institute of Physics.
Title: What can classical simulators learn from ab initio simulations?
Author(s): Delle Site L., Lynden-Bell R.M., Alavi A.
Journal Of Molecular Liquids, 98-9, pp. 79-86 (MAY 2002)
We compare the electrostatic properties of individual water molecules in ab initio simulations and in some potential models used in classical simulations. We find that the standard rigid water models at room temperature and ambient pressure describe the average electrostatic properties of molecules quite well, although there are considerable fluctuations. Preliminary results show that the dipole moment of water molecules near either a sodium ion or a methane molecule are not significantly different from its average value in bulk water, which may justify the use of non-polarisable water models in simulations of biological molecules. In ice, however, the dipole induced by the surroundings is appreciably higher. Incorrect freezing points can be attributed to a bad description of the solid phase rather than to a bad description of the liquid phase. (C) 2002 Elsevier Science B.V. All rights reserved.
Title: Tight-binding simulation of current-carrying nanostructures
Author(s): Todorov T.N.
Journal Of Physics-Condensed Matter, 14, No. 11, pp. 3049-3084 (MAR 25 2002)
The tight-binding (TB) approach to the modelling of electrical conduction in small structures is introduced. Different equivalent forms of the TB expression for the electrical current in a nanoscale junction are derived. The use of the formalism to calculate the current density and local potential is illustrated by model examples. A first-principles time-dependent TB formalism for calculating current-induced forces and the dynamical response of atoms is presented. An earlier expression for current-induced forces under steady-state conditions is generalized beyond local charge neutrality and beyond orthogonal TB. Future directions in the modelling of power dissipation and local heating in nanoscale conductors are discussed.
Title: Equilibrium structure of erbium-oxygen complexes in crystalline silicon
Author(s): Raffa A.G., Ballone P.
Physical Review B, 65, No. 12, Art. No. 121309 (MAR 15 2002)
The equilibrium structure of ErOn (nless than or equal to6) complexes in crystalline silicon has been investigated by density-functional computations. Two different geometries have been considered, corresponding to the substitutional and tetrahedral interstitial site for erbium. All atomic coordinates have been optimized by Car-Parrinello molecular dynamics. The resulting structures have low symmetry, with E-O distances of similar to2.35 Angstrom. The substitutional site is the most stable one for nless than or equal to2, while the tetrahedral interstitial is favored for n>2.
Title: Reactions of polycarbonate with cyclohexene oxide and phosphites: A density functional study
Author(s): Akola J., Ballone P., Jones R.O.
Macromolecules, 35, No. 6, pp. 2327-2334 (MAR 12 2002)
Epoxides and phosphites are often used as additives to stabilize the properties of polymers, including bisphenol A polycarbonate (BPA-PC). We describe density functional (DF) calculations of the reactions of cyclohexene oxide (CHO, cyclohexane epoxide) and phosphites with chain segments of BPA-PC, with the aim of identifying possible reaction paths and energy barriers. The reactions of CHO with the OH-terminated PC chains and with the carbonate group are exothermic, although there is an energy barrier in each case of more than 10 kcal/mol. A comparison of results for different CHO isomers demonstrates the importance of steric effects. The reactions between the same groups of the PC chain and the phosphites 2-[2,4-bis(tert-butyl)phenoxy]-5,5-dimethyl-1,3,2-dioxaphosphorinane] (BPDD) and trimethyl phosphite (TMP), and their phosphonate isomers are characterized by large energy barriers.
Title: Estimation of slow diffusion rates in confined systems: CCl4 in zeolite NaA
Author(s): Ghorai P.K.R., Yashonath S., Lynden-Bell R.M.
Molecular Physics, 100, No. 5, pp. 641-647 (MAR 2002)
Often the rate of passage of gaseous molecules through model zeolites is too small to be computed directly. An estimate for the rate of passage of CCl4 through the 8-ring window in a model of zeolite A has been obtained by combining a direct evaluation of the free energy profile and an adaptation of the rare events method. First the free energy profile is found from a direct evaluation of the canonical partition function at high dilution and the transition state theory rate constant obtained. The dynamic correction factor is then estimated from molecular dynamics runs and used to compute the actual rate k(eff). The method is used to estimate the rate of passage through the 8-ring window in a rigid model of zeolite A, and the results are compared with those obtained from rigid models with expanded windows and from the flexible model. Even a small expansion in the 8-ring window diameter increases the rate significantly, but the changes associated with a flexible cage are small.
Title: Power dissipation in nanoscale conductors
Author(s): Montgomery M.J., Todorov T.N., Sutton A.P.
Journal Of Physics-Condensed Matter, 14, No. 21, pp. 5377-5389 (JUN 3 2002)
A previous tight-binding model of power dissipation in a nanoscale conductor under an applied bias is extended to take account of the local atomic topology and the local electronic structure. The method is used to calculate the power dissipated at every atom in model nanoconductor geometries: a nanoscale constriction, a one-dimensional atomic chain between two electrodes with a resonant double barrier, and an irregular nanowire with sharp corners. The local power is compared with the local current density and the local density of states. A simple relation is found between the local power and the current density in quasiballistic geometries. A large enhancement in the power at special atoms is found in cases of resonant and anti-resonant transmission. Such systems may be expected to be particularly unstable against current-induced modifications.
Title: Path-integral study of a two-dimensional Lennard-Jones glass
Author(s): Ballone P., Montanari B.
Physical Review E, 65, No. 6, Art. No. 066704 (JUN 2002)
The glass transition in a quantum Lennard-Jones mixture is investigated by constant-volume path-integral simulations. Particles are assumed to be distinguishable, and the strength of quantum effects is varied by changing h from zero (the classical case) to one (corresponding to a highly quantum-mechanical regime). Quantum delocalization and zero point energy drastically reduce the sensitivity of structural and thermodynamic properties to the glass transition. Nevertheless, the glass transition temperature T-g can be determined by analyzing the phase space mobility of path-integral centroids. At constant volume, the T-g of the simulated model increases monotonically with increasing h. Low temperature tunneling centers are identified, and the quantum versus thermal character of each center is analyzed. The relation between these centers and soft quasilocalized harmonic vibrations is investigated. Periodic minimizations of the potential energy with respect to the positions of the particles are performed to determine the inherent structure of classical and quantum glassy samples. The geometries corresponding to these energy minima are found to be qualitatively similar in all cases. Systematic comparisons for ordered and disordered structures, harmonic and anharmonic dynamics, classical and quantum systems show that disorder, anharmonicity, and quantum effects are closely interlinked.
Title: Breather Scattering in the Dissipative Driven Frenkel-Kontorova Model
Author(s): Meister M.
Localization & Energy Transfer in Nonlinear Systems, ISBN 981-238-296-8, pp. 273-276 (JUN 2002)Abstract
The dissipative and driven Frenkel-Kontorova model supports moving breather solutions as attractors of the dynamics in a certain range of the system parameters. It is natural, then, to study the behaviour of such localised excitations in head-on collisions. The final state of a scattering process depends very sensitively on the initial conditions and can be qualitatively changed by very weak perturbations. At fixed values of the system parameters a larger number of initial conditions is used and from them several different bound states of two breathers are found. Bound states can be characterised by their velocity and by the distance between the breather cores. It turns out that certain values of this distance are preferred and that there also is a clear relation between the distance and the velocity.
Title: Reply to comment on 'Counterbalancing forces in electromigration'
Author(s): Hoekstra J., Sutton A.P., Todorov T.N.
Journal Of Physics-Condensed Matter, 14, No. 25, pp. 6603-6604 (JUL 1 2002)
Reply to comment by K-H W Chu.
Title: Americium(III) and europium(III) solvent extraction studies of amide-substituted triazine ligands and complexes formed with ytterbium(III)
Author(s): Boubals N., Drew M.G.B., Hill C., Hudson M.J., Iveson P.B., Madic C., Russell M.L., Youngs T.G.A.
Journal Of The Chemical Society-Dalton Transactions, No. 1, pp. 55-62 (JAN 7 2002)
4-Amino-bis(2,6-(2-pyridyl))-1,3,5-triazine L-4 and several, amide derivatives with hydrophobic alkyl substituents have been synthesised. Solvent extraction studies carried out on Am(III) and Eu(III) with L-4 and its amide derivatives in synergistic combination with alpha-bromodecanoic acid, show that these ligands can selectively extract actinides with respect to lanthanides. The structures of [H2L4].2Cl.2.5H(2)O and two amide derivatives have been determined and show respectively the trans, trans; cis, cis; and cis, trans conformations of the adjacent aromatic rings. These observed conformations are in agreement with the results of quantum mechanics calculations on L-4 and its protonated derivatives. The structures of two Yb complexes with amide derivatives are also reported with stoichiometry [Yb(L)(NO3)(H2O)(4)].2NO(3).0.5H(2)O and [Yb(L)(NO3)(3)(H2O)].2MeCN and show the metal in 9 coordinate environments.
Title: Model calculations for copper clusters on Au(111) surfaces
Author(s): Del Popolo M.G., Leiva E.P.M., Schmickler W.
Journal Of Electroanalytical Chemistry, 518, No. 2, pp. 84-90 (JAN 25 2002)
Using the semi-empirical embedded-atom method, the structure of small copper clusters on Au(111) surfaces has been investigated both by static and dynamic calculations. By varying the size of roughly circular clusters, the edge energy per atom is obtained; it agrees quite well with estimates based on experimental results. Small three-dimensional clusters tend to have the shape of a pyramid, whose sides are oriented in the directions of small surface energy. The presence of a cluster is found to distort the underlying lattice of adsorbed copper atoms. (C) 2002 Published by Elsevier Science B.V.
Title: Structures of adsorbed water layers on MgO: an ab initio study
Author(s): Lynden-Bell R.M., Delle Site L., Alavi A.
Surface Science, 496, No. 1-2, pp. L1-L6 (JAN 1 2002)
A systematic search using an ab initio density-functional method has been carried out for energy minima for a monolayer of water on MgO. Minima were sought in which one third of the water molecules were dissociated, and the observed p(3 x 2) symmetry satisfied. Six such minima were found, three of which are within 300k(B) per water molecule of the lowest energy structure. We also found a structure with a similar energy with (2 x 2) symmetry and half the water molecules dissociated. The structures are stabilised by the donation of three hydrogen bonds to each hydroxide ion. (C) 2001 Elsevier Science B.V. All rights reserved.
Title: NpT-ensemble Monte Carlo calculations for binary liquid mixtures - Comment
Author(s): Lynden-Bell R.M.
Molecular Physics, 100, No. 1, Art. No. 93 (JAN 2002)
Title: A molecular dynamics method for simulations in the canonical ensemble - Comment
Author(s): Lynden-Bell R.M.
Molecular Physics, 100, No. 1, Art. No. 189 (JAN 2002)
Title: Evidence for partial dissociation of water on flat MgO(100) surfaces
Author(s): Kim Y.D., Lynden-Bell R.M., Alavi A., Stulz J., Goodman D.W.
Chemical Physics Letters, 352, No. 5-6, pp. 318-322 (FEB 6 2002)
Ab initio calculations of the density of states for adsorbed monolayers of water on a MgO(100) surface are compared with metastable impact electron spectra (MIES) and ultraviolet photoelectron spectra (UPS). The calculations confirm the suggested assignments and in particular show that there is good evidence that some, but not all, molecules have dissociated to hydroxide ions. Density of state calculations are shown for configurations with no dissociated molecules, one third of the molecules dissociated and half the molecules dissociated; the agreement with experiment is best for the configuration with one third of the molecules dissociated. (C) 2002 Published by Elsevier Science B.V.
Title: Kinetics of phase transformations in a model with metastable fluid-fluid separation: A molecular dynamics study
Author(s): Costa D., Ballone P., Caccamo C.
Journal Of Chemical Physics, 116, No. 8, pp. 3327-3338 (FEB 22 2002)
By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid-fluid equilibrium, with a metastable fluid-fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76x10(6) MD steps. By progressively reducing the system temperature below the solid-fluid line, we first observe the metastable fluid-fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed. (C) 2002 American Institute of Physics.
Title: 2D-drop model applied to the calculation of step formation energies on a (111) substrate
Author(s): Rojas M.I., Amilibia G.E., Del Popolo M.G., Leiva E.P.M.
Surface Science, 499, No. 1, pp. L135-L140 (FEB 20 2002)
A model is presented for obtaining the step formation energy for metallic islands on (1 1 1) surfaces from Monte Carlo simulations. This model is applied to homo (Cu/Cu(1 1 1), Ag/Ag(1 1 1)) and heteroepitaxy (Ag/Pt(1 1 1)) systems. The embedded atom method is used to represent the interaction between the particles of the system, but any other type of potential could be used as well. The formulation can also be employed to consider the case of other single crystal surfaces, since the higher barriers for atom motion on other surfaces are not a hindrance for the simulation scheme proposed. (C) 2001 Elsevier Science B.V. All rights reserved.
Title: Counterbalancing forces in electromigration
Author(s): Hoekstra J., Sutton A.P., Todorov T.N.
Journal Of Physics-Condensed Matter, 14, No. 6, pp. L137-L140 (FEB 18 2002)
In electromigration (EM) experiments on metallic wires, a flux of atoms can lead to motion of the centre of mass (COM) of the wire. Hence, it may be tempting to assume that the flow of current produces a net force on the wire as a whole. We point out, on the basis of known momentum-balance arguments, that the net force on a metallic wire due to a passing steady-state current is zero. This is possible, because in addition to EM driving forces, acting on scattering centres, there are counterbalancing forces, acting on the rest of the system. Drift of the COM in EM experiments occurs inevitably because the substrate keeps the crystal lattice of the wire fixed, while allowing diffusion of defects in the bulk of the wire. This drift is not evidence for a net force on the wire.
Title: Efficient grid treatment of the ionization dynamics of laser-driven H-2(+)
Author(s): Dundas D.
Physical Review A, 65, No. 2, Art. No. 023408 (FEB 2002)
We implement a parallel, time-dependent hybrid finite-difference Lagrange mesh code to model the electron dynamics of the fixed-nuclei hydrogen molecular ion subjected to intense ultrashort laser Pulses, Ionization rates are calculated and compared with results from a previous finite-difference approach and also with published Floquet results. The sensitivity of the results to the gauge describing the electron-field interaction is studied. Visualizations of the evolving wave packets are also presented in which the formation of a stable bound-state resonance is observed.
Title: High-pressure deformation mechanism in scolecite: A combined computational-experimental study
Author(s): Ballone P., Quartieri S., Sani A., Vezzalini G.
American Mineralogist, 87, No. 8-9, pp. 1194-1206 (AUG-SEP 2002)Abstract
Pressure-induced structural modifications in scolecite were studied by means of in situ synchrotron X-ray powder diffraction and density functional computations. The experimental cell parameters were refined up to 8.5 GPa. Discontinuities in the slope of the unit-cell parameters vs. pressure dependence were observed; as a consequence, an increase in the slope of the linear pressure-volume dependence is observed at about 6 GPa, suggesting an enhanced compressibility at higher pressures. Weakening and broadening of the diffraction peaks reveals increasing structural disorder with pressure, preventing refinement of the lattice parameters above 8.5 GPa. Diffraction patterns collected during decompression show that the disorder is irreversible. Atomic coordinates within unit cells of different dimensions were determined by means of Car-Parrinello simulations. The discontinuous rise in compressibility at about 6 GPa is reproduced by the computation, allowing us to attribute it to re-organization of the hydrogen bonding network, with the formation of water dimers. Moreover we found that, with increasing pressure, the tetrahedral chains parallel to c rotate along their elongation axis and display an increasing twisting along a direction perpendicular to c. At the same time, we observed the compression of the channels. We discuss the modification of the Ca polyhedra under pressure, and the increase in coordination number (from 4 to 5) of one of the two Al atoms, resulting from the approach of a water molecule. We speculate that this last transformation triggers the irreversible disordering of the system.
Title: Silicon segregation from SiO (2-X) sub-oxides: Dynamical Monte Carlo study.
Author(s): Ballone P., Jones R.O.
Abstracts Of Papers Of The American Chemical Society, 224, Art. No. U475 (AUG 18 2002)Abstract
Title: Accurate and efficient many-body potential for hydrogen bonding - Application to water.
Author(s): Ballone P., Akola J., Jones R.O.
Abstracts Of Papers Of The American Chemical Society, 224, Art. No. U482 (AUG 18 2002)Abstract
Title: Electronic structure and chemical potentials of molecules in spc/e water.
Author(s): Lynden-Bell R.M., Murdock S.
Abstracts Of Papers Of The American Chemical Society, 224, Art. No. U501 (AUG 18 2002)Abstract
Title: Simulation studies of solvation in imidazolium ionic liquids.
Author(s): Lynden-Bell R.M., Hanke C.
Abstracts Of Papers Of The American Chemical Society, 224, Art. No. U619 (AUG 18 2002)Abstract
Title: Quantum conductance of multiwall carbon nanotubes
Author(s): Yoon Y.G., Delaney P., Louie S.G.
Physical Review B, 66, No. 7, Art. No. 073407 (AUG 15 2002)
An analysis on the conductance of multiwall carbon nanotubes (MWNT's) is presented. Recent experiment indicated that MWNT's are good quantum conductors. Our theory shows that tunneling current between states on different walls of a defect-free, infinitely long MWNT is vanishingly small in general, which leads to the quantization of the conductance of the MWNT's. With a reasonable simple model, we explicitly show that the conductance of a capped MWNT can be determined by the outermost wall for an infinitely long nanotube. We apply the theory to finite MWNT's and estimate the generic interwall conductance to be negligible compared to the intrawall ballistic conductance.
Title: Density functional/Monte Carlo study of ring-opening polymerization
Author(s): Ballone P., Jones R.O.
Computer Physics Communications, 147, No. 1-2, pp. 325-330 (AUG 1 2002)
Density functional calculations of the structure, potential energy surface and reactivity for organic systems closely related to bisphenol-A-polycarbonate (BPA-PC) provide the basis for a model describing the ring-opening polymerization of its cyclic oligomers by nucleophilic molecules. Monte Carlo simulations using this model show a strong tendency to polymerize that is increased by increasing density and temperature, and is greater in 3D than in 2D. Entropy in the distribution of inter-particle bonds is the driving force for chain formation. (C) 2002 Elsevier Science B.V. All rights reserved.
Title: Finite element modeling of the morphology of beta to alpha phase transformation in Ti-6Al-4V alloy
Author(s): Katzarov I., Malinov S., Sha W.
Metallurgical And Materials Transactions A-Physical Metallurgy And Materials Science, 33, No. 4, pp. 1027-1040 (APR 2002)
In the present work, a mathematical model and computer programs were developed for numerical simulation of the processes of nucleation and growth of the alpha-phase Widmanstatten plates during the course of the beta double right arrow alpha phase transformation in a Ti-6Al-4V alloy. The alpha-phase appearance at the grain boundary of beta phase is described by a numerical procedure for random nucleation as a function of the vanadium concentration and the temperature. The rate at which an interface moves depends both on the intrinsic mobility and on the rate at which diffusion can remove the excess of vanadium atoms ahead of the interface. The finite-element method (FEM) was used for solving the diffusion equation on the domain occupied by beta phase. The elements chosen have dimensions in both space and time. A computer code based on the finite-element modeling and the volume of fluids method was developed to trace the movement of the alpha/beta interface. The influences of the cooling rate and the temperature of isothermal exposure on the Widmanstatten morphology were simulated and analyzed. The developed models and program packages are capable of one-dimensional (1-D) and two-dimensional (2-D) simulations of the morphology of the beta double right arrow alpha phase transformation in Ti-6Al-4V alloy for continuous cooling with any cooling path and for an arbitrary combination between continuous cooling and isothermal exposure.
Title: Solvation of small molecules in imidazolium ionic liquids: a simulation study
Author(s): Hanke C.G., Atamas N.A., Lynden-Bell R.M.
Green Chemistry, 4, No. 2, pp. 107-111 (APR 2002)
Molecular dynamics simulations of a number of small molecules dissolved in dimethylimidazolium chloride at 400 K have been performed. The molecules chosen were water, methanol, dimethyl ether and propane, which have a range of properties from polar and hydrogen-bonding to non-polar. The local structure was analysed through ranked radial distribution functions and three dimensional probability functions, and the interaction energy with the anions and cations determined. The energetics and local structure show that the strongest interactions are hydrogen bonding to the chloride ion for water and methanol, while dimethyl ether and propane interact more strongly with the cation.
Title: Theoretical considerations of electrochemical phase formation for an ideal Frank-van der Merwe system - Ag on Au(111) and Au(100)
Author(s): Gimenez M.C., Del Popolo M.G., Leiva E.P.M., Garcia S.G., Salinas D.R., Mayer C.E., Lorenz W.J.
Journal Of The Electrochemical Society, 149, No. 4, pp. E109-E116 (APR 2002)
Static calculation and preliminary kinetic Monte Carlo simulation studies are undertaken for the nucleation and growth on a model system which follows a Frank-van der Merwe mechanism. In the present case, we consider the deposition of Ag on Au(100) and Au(111) surfaces. The interactions were calculated using the embedded atom model. The kinetics of formation and growth of 2D Ag structures on Au(100) and Au(111) is investigated and the influence of surface steps on this phenomenon is studied. Very different time scales are predicted for Ag diffusion on Au(100) and Au(111), thus rendering very different regimes for the nucleation and growth of the related 2D phases. These observations are drawn from the application of a model free of any adjustable parameter. (C) 2002 The Electrochemical Society.
Title: A stabilization mechanism of zirconia based on oxygen vacancies only
Author(s): Fabris S., Paxton A.T., Finnis M.W.
Acta Materialia, 50, No. 20, pp. 5171-5178 (2002)
The microscopic mechanism leading to stabilization of cubic and tetragonal forms of zirconia (ZrO2) is analyzed by means of a self-consistent tight-binding model. Using this model, energies and structures of zirconia containing different vacancy concentrations are calculated, equivalent in concentration to the charge compensating vacancies associated with dissolved yttria (Y2O3) in the tetragonal and cubic phase fields (3.2 and 14.4% mol, respectively). The model is shown to predict the large relaxations around an oxygen vacancy, and the clustering of vacancies along the 111 directions, in good agreement with experiments and first principles calculations. The vacancies alone are shown to explain the stabilization of cubic zirconia, and the mechanism is analyzed. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
Title: Solvation effects on equilibria: Triazoles and N-methyl piperidinol
Author(s): Murdock S.E., Lynden-Bell R.M., Kohanoff J., Margulis C.J., Sexton G.J.
Physical Chemistry Chemical Physics, 4, No. 21, pp. 5281-5288 (2002)
doi: 10.1039/b203429h 10.1039/b203429h
We have performed calculations of the solvation effects on a number of equilibrium constants in water using a recently proposed hybrid quantum classical scheme in which the liquid environment is modelled using classical solvent molecules and the solute electronic structure is computed using modern quantum chemical methods. The liquid phase space is sampled from a fully classical simulation. We find that solvation effects on both triazole tautomeric equilibrium constants and piperidinol conformational equilibrium constants can be interpreted in terms of subtle differences in the local environment which can be seen in probability densities and radial distribution functions. Lower level calculations were performed for comparison and we conclude that the solvation thermodynamics can be predicted from a good classical model of solvent and solute molecules, but the implicit models that we tried are less successful.
Title: Electron energy-loss near-edge shape as a probe to investigate the stabilization of yttria-stabilized zirconia
Author(s): Ostanin S., Craven A.J., McComb D.W., Vlachos D., Alavi A., Paxton A.T., Finnis M.W.
Physical Review B, 65, No. 22, Art. No. 224109 (2002)
The electron energy-loss near-edge structure (ELNES) at the O K edge has been studied in yttria-stabilized zirconia (YSZ). The electronic structure of YSZ for compositions between 3 and 15 mol % Y2O3 has been computed using a pseudopotential-based technique to calculate the local relaxations near the O vacancies. The results showed phase transition from the tetragonal to cubic YSZ at 10 mol % of Y2O3, reproducing experimental observations. Using the relaxed defect geometry, calculation of the ELNES was carried out using the full-potential linear muffin-tin orbital method. The results show very good agreement with the experimental O K-edge signal, demonstrating the power of using ELNES to probe the stabilization mechanism in doped metal oxides.
Title: On the relevance of tunneling for the isotope effect in KDP
Author(s): Koval S., Kohanoff J., Migoni R.L.
Ferroelectrics, 268, pp. 659-664 (2002)Abstract
First-principles calculations for KH2PO4 (KDP) show that hydrogen off-center (OC) ordering in the ferroelectric phase is accompanied by electronic charge delocalization from the acceptor and localization at the donor oxygen within the O-H...O bonds. This induces P atoms off-centering in the PO4 tetrahedrons and pairing with K+ ions along the tetragonal axis, thus developing macroscopic polarization. Centered H are collectivelly unstable. However, if the protons are centered, the P and K atoms are stable in their centered positions. In addition, off-centering of a single proton is not energetically favorable. Only correlated proton motions, involving also displacements of heavier atoms, exhibit significant double wells, thus questioning the role of tunneling in isotope effects.
Title: Determining the electronic structure and chemical potentials of molecules in solution
Author(s): Murdock S.E., Lynden-Bell R.M., Kohanoff J., Sexton G.J.
Physical Chemistry Chemical Physics, 4, No. 13, pp. 3016-3021 (2002)
doi: 10.1039/b200991a 10.1039/b200991a
A simulation scheme is proposed for determining the excess chemical potential of a substance in solution. First, a Monte Carlo simulation is performed with classical models for solute and solvent molecules. A representative sample of these configurations is then used in a hybrid quantum/classical (QM/MM) calculation, where the solute is treated quantum-mechanically, and the average electronic structure is used to construct an improved classical model. This procedure is iterated to self-consistency in the classical model, which in practice is attained in one or two steps, depending on the quality of the initial guess. The excess free energy of the molecule within the QM/MM approach is determined relative to the classical model using thermodynamic perturbation theory with a cumulant expansion. The procedure provides a method of constructing classical point charge models appropriate for the solution and gives a measure of the importance of solvent fluctuations.
Title: The Failure of Generalized Gradient Approximations (GGAs) and Meta-GGAs for the Two-Center Three-Electron Bonds in He2+, (H2O)2+, and (NH3)2+
Author(s): Gruening M., Gritsenko O., van Gisbergen S., Baerends E.,
Journal of Chemical Physics A, 105, pp. 9211- (2002)Abstract
Title: On the required shape corrections to the local density and generalized gradient approximations to the Kohn–Sham potentials for molecular response calculations of (hyper)polarizabilities and excitation energies
Author(s): Gruening M., Gritsenko O., van Gisbergen S., Baerends E,
Journal of Chemical Physics, 116, pp. 9591- (2002)Abstract
Title: Exchange potential from the common energy denominator approximation for the Kohn–Sham Green’s function: Application to (hyper)polarizabilities of molecular chains
Author(s): Gruening M., Gritsenko O., Baerends.,
Journal of Chemistry Physics, 116, pp. 6435- (2002)Abstract