Recent Publications

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  1. Title: An isothermal-isobaric Langevin thermostat for simulating nanoparticles under pressure: Application to Au clusters

    Author(s): Kohanoff J., Caro A., Finnis M.W.

    Chemphyschem, 6, No. 9, pp. 1848-1852 (SEP 5 2005)

    doi: 10.1002/cphc.200400607

    We present a method for simulating clusters or, molecules subjected to an external pressure, which is exerted by a pressure-transmitting medium. It is based on the canoninical Langevin thermostat, but extended in such a way that the Brownian forces are allowed to operate only from the region exterior to the cluster. We show that the frictional force of the Langevin thermostat is linked to the pressure of the reservoir in a unique way, and that this property manifests itself when the particle it acts upon is not pointlike but has finite dimensions. By choosing appropriately the strength of the random forces and the friction coefficient, both temperature and pressure can be controlled independently. We illustrate the capabilities of this new method by calculating the compressibility of small gold clusters under pressure.

  2. Title: Structure and dynamics of a confined ionic liquid. topics of relevance to dye-sensitized solar cells

    Author(s): Pinilla C., Del Popolo M.G., Lynden-Bell R.M., Kohanoff J.

    Journal Of Physical Chemistry B, 109, No. 38, pp. 17922-17927 (SEP 29 2005)

    doi: 10.1021/jp052999o

    The behavior of a model ionic liquid (IL) confined between two flat parallel walls was studied at various interwall distances using computer simulations. The results focus both on structural and dynamical properties. Mass and charge density along the confinement axis reveal a structure of layers parallel to the walls that leads to an oscillatory profile in the electrostatic potential. Orientational correlation functions indicate that cations at the interface orient tilted with respect to the surface and that any other orientational order is lost thereafter. The diffusion coefficients of the ions exhibit a maximum as a function of the confinement distance, a behavior that results from a combination of the structure of the liquid as a whole and a faster molecular motion in the vicinity of the walls. We discuss the relevance of the present results and elaborate on topics that need further attention regarding the effects of ILs in the functioning of IL-based dye-sensitized solar cells.

  3. Title: Current-driven magnetic rearrangements in spin-polarized point contacts

    Author(s): Stamenova M., Sanvito S., Todorov T.N.

    Physical Review B, 72, No. 13, Art. No. 134407 (OCT 2005)

    doi: 10.1103/PhysRevB.72.134407

    A method for investigating the dynamics of atomic magnetic moments in current-carrying magnetic point contacts under bias is presented. This combines the nonequilibrium Green's function (NEGF) method for evaluating the current and the charge density with a description of the dynamics of the magnetization in terms of quasistatic thermally activated transitions between stationary configurations. This method is then implemented in a tight-binding (TB) model with parameters chosen to simulate the main features of the electronic structures of magnetic transition metals. We investigate the domain wall (DW) migration in magnetic monoatomic chains sandwiched between magnetic leads, and for realistic parameters find that collinear arrangement of the magnetic moments of the chain is always favorable. Several stationary magnetic configurations are identified, corresponding to a different number of Bloch walls in the chain and to a different current. The relative stability of these configurations depends on the geometrical details of the junction and on the bias; however, we predict transitions between different configurations with activation barriers of the order of a few tens of meV. Since different magnetic configurations are associated with different resistances, this suggests an intrinsic random telegraph noise at microwave frequencies in the I-V curves of magnetic atomic point contacts at room temperature. Finally, we investigate whether or not current-induced torques are conservative.

  4. Title: Automatic generation of matrix element derivatives for tight binding models

    Author(s): Elena A.M., Meister M.

    Physical Review B, 72, No. 16, Art. No. 165107 (OCT 2005)

    doi: 10.1103/PhysRevB.72.165107

    Full Text

    Tight binding (TB) models are one approach to the quantum mechanical many-particle problem. An important role in TB models is played by hopping and overlap matrix elements between the orbitals on two atoms, which of course depend on the relative positions of the atoms involved. This dependence can be expressed with the help of Slater-Koster parameters, which are usually taken from tables. Recently, a way to generate these tables automatically was published. If TB approaches are applied to simulations of the dynamics of a system, also derivatives of matrix elements can appear. In this work we give general expressions for first and second derivatives of such matrix elements. Implemented in a tight binding computer program, like, for instance, DINAMO, they obviate the need to type all the required derivatives of all occurring matrix elements by hand.

  5. Title: Expressions for the second-order mixed partial derivatives of Slater-Koster matrix elements at spherical coordinate singularities

    Author(s): Meister M.

    Physical Review B, 72, No. 16, Art. No. 165108 (OCT 2005)

    doi: 10.1103/PhysRevB.72.165108

    In a recent publication it has been shown how to generate derivatives with respect to atom coordinates of Slater-Koster matrix elements for the tight binding modeling of a system. For the special case of a mixed second partial derivative at coordinate singularities only the results were stated in that publication. In this work, the derivation of these results is given in detail. Though it may seem rather technical and only applicable to a very special case, atomic configurations where the connecting vector between the two atoms involved in a two-center matrix element is aligned along the z axis (in the usual approach) require results for precisely this case. The expressions derived in this work have been implemented in the DINAMO code.

  6. Title: First-principles study of ferroelectricity and isotope effects in H-bonded KH2PO4 crystals

    Author(s): Koval S., Kohanoff J., Lasave J., Colizzi G., Migoni R.L.

    Physical Review B, 71, No. 18, Art. No. 184102 (MAY 2005)

    doi: 10.1103/PhysRevB.71.184102

    By means of extensive first-principles calculations we studied the ferroelectric phase transition and the associated isotope effect in KH2PO4 (KDP). Our calculations revealed that the spontaneous polarization of the ferroelectric phase is due to electronic charge redistributions and ionic displacements which are a consequence of proton ordering, and not vice versa. The experimentally observed double-peaked proton distribution in the paraelectric phase cannot be explained by a dynamics of only protons. This requires, instead, collective displacements within clusters that include also the heavier ions. These tunneling clusters can explain the recent evidence of tunneling obtained from Compton scattering measurements. The sole effect of mass change upon deuteration is not sufficient to explain the huge isotope effect. Instead, we find that structural modifications deeply connected with the chemistry of the H bonds produce a feedback effect on tunneling that strongly enhances the phenomenon. The resulting influence of the geometric changes on the isotope effect agrees with experimental data from neutron scattering. Calculations under pressure allowed us to analyze the issue of universality in the disappearance of ferroelectricity upon compression. Compressing DKDP so that the distance between the two peaks in the deuteron distribution is the same as for protons in KDP, corresponds to a modification of the underlying double-well potential, which becomes 23 meV shallower. This energy difference is what is required to modify the O-O distance in such a way as to have the same distribution for protons and deuterons. At the high pressures required experimentally, the above feedback mechanism is crucial to explain the magnitude of the geometrical effect.

  7. Title: Ab initio molecular dynamics simulation of a room temperature ionic liquid

    Author(s): Del Popolo M.G., Lynden-Bell R.M., Kohanoff J.

    Journal Of Physical Chemistry B, 109, No. 12, pp. 5895-5902 (MAR 31 2005)

    doi: 10.1021/jp044414g

    Ab initio molecular dynamics simulations have been performed for the first time on the room-temperature organic ionic liquid dimethyl imidazolium chloride [DMIM][Cl] using density functional theory. The aim is to compare the local liquid structure with both that obtained from two different classical force fields and from neutron scattering experiments. The local structure around the cation shows significant differences compared to both the classical calculations and the neutron results. In particular, and unlike in the gas-phase ion pair, chloride ions tend to be located near a ring C-H proton in a position suggesting hydrogen bonding. The results are used to suggest ways in which the classical potentials may be improved.

  8. Title: Vibrational properties of nanometric AB(2) ionic clusters

    Author(s): Montanari B., Ballone P., Mazza T., Milani P.

    Journal Of Physics-Condensed Matter, 17, No. 25, pp. 3787-3806 (JUN 29 2005)

    doi: 10.1088/0953-8984/17/25/006

    A broad survey of harmonic dynamics in AB(2) clusters with up to N = 3000 atoms is performed using a simple rigid ion model, with ionic radii selected to give rutile as the ground state structure for the corresponding extended crystal. The vibrational density of states is already close to its bulk counterpart for N similar to 500, with characteristic differences due to surfaces, edges and vertices. Two methods are proposed and tested to map the cluster vibrational states onto the rutile crystal phonons. The net distinction between infrared (IR) active and Raman active modes that exists for bulk rutile becomes more and more blurred as the cluster size is reduced. It is found that, in general, the higher the IR activity of the mode, the more this is affected by the system size. IR active modes are found to spread over a wide frequency range for the finite clusters. Simple models based on either a crude confinement constraint or surface pressure arguments fail to reproduce the results of the calculations. The effects of the stoichiometry and dielectric properties of the surrounding medium on the vibrational properties of the clusters are also investigated.

  9. Title: Correlated electron-ion dynamics with open boundaries: formalism

    Author(s): Bowler D.R., Horsfield A.P., Sanchez C.G., Todorov T.N.

    Journal Of Physics-Condensed Matter, 17, No. 25, pp. 3985-3995 (JUN 29 2005)

    doi: 10.1088/0953-8984/17/25/024

    We extend a new formalism, which allows correlated electron-ion dynamics to be applied to the problem of open boundary conditions. We implement this at the first moment level (allowing heating of ions by electrons) and observe the expected cooling in the classical part of the ionic kinetic energy and current-induced heating in the quantum contribution. The formalism for open boundaries should be easily extended to higher moments of the correlated electron-ion fluctuations.

  10. Title: Effects of tip structure on the generation of metal clusters by an STM tip: a way to control the orientation of nanocrystallites?

    Author(s): Mariscal M., Narambuena C.F., Del Popolo M.G., Leiva E.P.M.

    Nanotechnology, 16, No. 6, pp. 974-980 (JUN 2005)

    doi: 10.1088/0957-4484/16/6/059

    The role of the crystalline orientation of the STM tip in the generation of metal clusters is studied by atom dynamics simulations. When a (111) facet is facing the surface, the process is accompanied by a perturbation of the surface stronger than that observed for more open tip structures. This implies a technological application: the possibility of orienting a nanocrystallite deposited on a tip according to the changes observed in the force on the tip.

  11. Title: Multiphoton double ionization of atoms and molecules by FEL XUV light

    Author(s): Taylor K.T., Parker J.S., Dundas D., Meharg K.J., Doherty B.J.S., Murphy D.S., McCann J.F.

    Journal Of Electron Spectroscopy And Related Phenomena, 144, No. Sp. Iss. SI, pp. 1191-1196 (JUN 2005)

    doi: 10.1016/j.elspec.2005.01.270

    We review recent work carried out in Belfast which handles the few-electron dynamics of atomic and molecular systems exposed to high frequency, high intensity laser fields. The design and application of the quantitatively accurate computational methods is discussed. The Belfast work is illustrated by results for double ionization of helium and the hydrogen molecule by FEL XUV light where in each case the two electrons have been handled in full-dimensionality. (c) 2005 Elsevier B.V. All rights reserved.

  12. Title: Mathematical modeling and numerical simulation of the formation and growth of a two-phase layer during diffusion in ternary system

    Author(s): Katzarov I., Malinov S., Yanakieva V.

    Acta Materialia, 53, No. 10, pp. 3091-3099 (JUN 2005)

    doi: 10.1016/j.actamat.2005.03.032

    A system of partial differential equations was derived to describe diffusion in the two-phase region of a ternary alloy by assuming that the mixture concentration in an infinitesimally small volume can be changed by external fluxes over the boundaries of this volume. A numerical procedure was developed for solving the system of partial differential equation (PDE) and treating the problem with the moving boundaries between various single-phase or two-phase layers. The mathematical model was applied for numerical simulation of the formation and growth of the single-phase and two-phase layers, developed in ternary alloy specimen, through diffusion from the surface to the core. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  13. Title: Optimization by quantum annealing: Lessons from simple cases

    Author(s): Stella L., Santoro G.E., Tosatti E.,

    Physical Review B, 72, No. 1, Art. No. 014303 (July 2005)

    doi: 10.1103/PhysRevB.72.014303

    We investigate the basic behavior and performance of simulated quantum annealing (QA) in comparison with classical annealing (CA). Three simple one-dimensional case study systems are considered: namely, a parabolic well, a double well, and a curved washboard. The time-dependent Schrodinger evolution in either real or imaginary time describing QA is contrasted with the Fokker-Planck evolution of CA. The asymptotic decrease of excess energy with annealing time is studied in each case, and the reasons for differences are examined and discussed. The Huse-Fisher classical power law of double-well CA is replaced with a different power law in QA. The multiwell washboard problem studied in CA by Shinomoto and Kabashima and leading classically to a logarithmic annealing even in the absence of disorder turns to a power-law behavior when annealed with QA. The crucial role of disorder and localization is briefly discussed.

  14. Title: Interactions of triiodide cluster ion with solvents

    Author(s): Zhang F.S., Lynden-Bell R.M.

    European Physical Journal D, 34, No. 1-3, pp. 129-132 (JUL 2005)

    doi: 10.1140/epjd/e2005-00133-6

    An equilibrium molecular dynamics model is developed to investigate the interactions of triiodide cluster ion with solvents. The internal dynamics of the triiodide ion is described by a valence bond model which responds to the field of the classical solvent molecules. The solvent molecules were described by standard classical models with rigid molecules, fixed partial charges on atomic sites and site-site Lennard-Jones interactions. One finds the solvent effects on the I-3(-) are unusually strong as it is a very polarizable species. Protic solvents such as water, ethanol, and methanol that can form hydrogen bonds to lead to the I-3(-) geometry with two unequal bonds and an asymmetric distribution of charges. But for the solvents such as xenon, tetrahydrofuran, methyltetrahydrofuran, and acetonitrile, the I-3(-) only illustrates a geometry with two equal bonds. We find that structure changing is induced by local electrostatic attraction between solvent molecules.

  15. Title: Synthesis and characterization of fluorescent poly(aromatic amide) dendrimers

    Author(s): Aulenta F., Drew M.G.B., Foster A., Hayes W., Rannard S., Thornthwaite D.W., Worrall D.R., Youngs T.G.A.

    Journal Of Organic Chemistry, 70, No. 1, pp. 63-78 (JAN 7 2005)

    doi: 10.1021/jo048799a

    The synthesis of a series of poly(aromatic amide) dendrimers up to the second generation is described herein. The AB, building block used throughout the synthesis of the dendrimers was the allyl ester of 3,5-diaminocinnamic acid, which has been synthesized from 3,5-dinitrobenzoic acid in good yield with use of a four-step procedure. Dendron synthesis was achieved via a convergent approach with use of a sequence of deprotection/coupling steps. Two commercially available alcohols, L-menthol and citronellol, were coupled to the AB(2) monomer by using an alkyl diacid spacer and two core units; 1,7-diaminoheptane and tris(2-aminoethyl)amine have been used to produce the final dendrimers. Characterization was carried out by NMR and IR spectroscopies, MALDI-TOF mass spectrometry, GPC, and DSC. The novel monomer and dendritic derivatives exhibited a strong fluorescence emission in the visible region (lambda approximate to 500 nm) of the spectrum and a weak emission in the near-infrared (lambda approximate to 850 nm) upon excitation in the near-UV region. The fluorescence emission characteristics were found to be solvent and dendrimer generation dependent.

  16. Title: Symmetry, delocalization, and molecular conductance

    Author(s): Delaney P., Nolan M., Greer J.C.

    Journal Of Chemical Physics, 122, No. 4, Art. No. 044710 (JAN 22 2005)

    doi: 10.1063/1.1836754

    Molecules bonded between two metal contacts form the simplest possible molecular devices. Coupled by the molecule, the left and right contact-based states form symmetric and antisymmetric pairs near the Fermi level. We relate the size of the resulting energy splitting DeltaE to the symmetry and degree of delocalization of the coupling molecular orbital. Qualitative trends in molecular conductances are then estimated from the variations in DeltaE. We examine benzenedithiol and other molecules of interest in transport. (C) 2005 American Institute of Physics.

  17. Title: Fragrance release from the surface of branched poly(amide)s

    Author(s): Aulenta F., Drew M.G.B., Foster A., Hayes W., Rannard S., Thornthwaite D.W., Youngs T.G.A.

    Molecules, 10, No. 1, pp. 81-97 (JAN 2005)


    Full Text

    Enzymes are powerful tools in organic synthesis that are able to catalyse a wide variety of selective chemical transformations under mild and environmentally friendly conditions. Enzymes such as the lipases have also found applications in the synthesis and degradation of polymeric materials. However, the use of these natural catalysts in the synthesis and the post-synthetic modification of dendrimers and hyperbranched molecules is an application of chemistry yet to be explored extensively. In this study the use of two hydrolytic enzymes, a lipase from Candida cylindracea and a cutinase from Fusarium solani pisii, were investigated in the selective cleavage of ester groups situated on the peripheral layer of two families of branched polyamides. These branched polyamides were conjugated to simple fragrances citronellol and L-menthol via ester linkages. Hydrolysis of the ester linkage between the fragrances and the branched polyamide support was carried out in aqueous buffered systems at slightly basic pH values under the optimum operative conditions for the enzymes used. These preliminary qualitative investigations revealed that partial cleavage of the ester functionalities from the branched polyamide support had occurred. However, the ability of the enzymes to interact with the substrates decreased considerably as the branching density, the rigidity of the structure and the bulkiness of the polyamide-fragrance conjugates increased.

  18. Title: Molecular effects in the ionization of N2, O2, and F2 by intense laser fields

    Author(s): Dundas D., Rost J.-M.

    Physical Review A, 71, No. 1, Art. No. 013421 (JAN 2005)

    doi: 10.1103/PhysRevA.71.013421

    In this paper we study the response in time of N2, O2, and F2 to laser pulses having a wavelength of 390 nm. We find single-ionization suppression in O2 and its absence in F2, in accordance with experimental results at λ=800 nm. Within our framework of time-dependent density functional theory we are able to explain deviations from the predictions of intense-field many-body S-matrix theory (IMST). We confirm the connection of ionization suppression with destructive interference of outgoing electron waves from the ionized electron orbital. However, the prediction of ionization suppression, justified within the IMST approach through the symmetry of the highest occupied molecular orbital (HOMO), is not reliable since it turns out that—e.g., in the case of F2—the electronic response to the laser pulse is rather complicated and does not lead to dominant depletion of the HOMO. Therefore, the symmetry of the HOMO is not sufficient to predict ionization suppression. However, at least for F2, the symmetry of the dominantly ionized orbital is consistent with the nonsuppression of ionization.

  19. Title: Solvent-induced symmetry breaking: Varying solvent strength

    Author(s): Zhang F.S., Lynden-Bell R.M.

    Physical Review E, 71, No. 2, Art. No. 021502 (FEB 2005)

    doi: 10.1103/PhysRevE.71.021502

    The triiodide ion is an example of a system where symmetry breaking may be induced by a solvent. The Landau free energy is expected to have a similar form to that for the mean field Ising model, but with solvent strength rather than temperature as the control parameter determining whether there is symmetry breaking. In order to examine the extent of anomalous behavior near the critical point we have studied the properties of the ion in a solvent based on a model for water with charges scaled by a factor lambda. As lambda is increased from zero the solvent strength increases and the system changes from one with no symmetry breaking to one with strong symmetry breaking. The Shannon entropy as a function of lambda shows only a weak maximum near the critical value of lambda=lambda(c), while the susceptibility has no anomalous behavior. We examine a simple model and show that anomalies near the critical point would increase as the temperature decreases, but divergences associated with a second order phase transition would only be seen in the limit of zero temperature.

  20. Title: Arrow diagram approach to nonorthogonal electron group functions in extended systems

    Author(s): Wang Y., Kantorovich L.

    Journal of Physics: Condensed Matter, 18, pp. 295-313 (DEC 9 2005)

    doi: 10.1088/0953-8984/18/1/022

    Abstract. For nonorthogonal electron group functions, the arrow diagram (AD) method (Kantorovich and Zapol 1992 J. Chem. Phys. 96 8420; 1992 J. Chem. Phys. 96 8427) provides a convenient procedure for calculating matrix elements \left \langle \Psi \right |\widehat {O}\left |\Psi \right \rangle of arbitrary symmetrical operators \widehat {O} . The total wavefunction of the system \Psi=\widehat {A}\prod_{I}\Phi _{I} is represented as an antisymmetrized product of nonorthogonal many-electron group functions ΦI of each group I in the system. For extended (e.g. infinite) systems the calculation of the mean value of an operator is ill defined, however, as it requires that each term of the diagram expansion be divided by the normalization integral S=\left \langle \Psi \vert \Psi \right \rangle which is given by an AD expansion as well. In this work, we cast the mean value of a symmetrical operator in a form of an AD expansion which is a linear combination of linked ADs. By analysing an exactly solvable one-dimensional Hartree–Fock problem, we find that pre-factors, attached to every linked AD in the linear combination, can be expanded in a power series with respect to overlap. A general method of calculating these pre-factors in a form of a power series expansion with respect to overlap is suggested. This advance makes the AD theory applicable to extended systems, and allows one to calculate the mean value of an arbitrary symmetrical operator correct up to the desired order of overlap within the group function theory. In particular, we derive the effective Hamiltonian of a quantum cluster surrounded by overlapping group functions (e.g. bonds) in the environment region which is correct up to the second order with respect to overlap (an embedding problem).

  21. Title: On the generation of metal clusters with the electrochemical scanning tunneling microscope

    Author(s): Del Popolo M.G., Leiva E.P.M., Mariscal M., Schmickler W.

    Surface Science, 597, No. 1-3, pp. 133-155 (DEC 15 2005)

    doi: 10.1016/j.susc.2004.04.063

    In the present work we consider two aspects of the deposition of metal clusters on an electrode surface. The formation of such clusters with the tip of a scanning tunneling microscope is simulated by atom dynamics. Subsequently the stability of these clusters is investigated by Monte Carlo simulations in a grand-canonical ensemble. In particular, the following systems were considered explicitly: Pd clusters on Au(111), Cu on Au(111), Ag on Au(111), Pb on Au(111) and Cu on Ag(111). The analysis of the results obtained for the different systems leads to the conclusion that optimal systems for nanostructuring are those where the metals participating have similar cohesive energies and negative heats of alloy formation. In this respect, the system Cu-Pd(111) is predicted as a good candidate for the formation of stable clusters. (c) 2005 Elsevier B.V. All rights reserved.

  22. Title: Correlated electron-ion dynamics: the excitation of atomic motion by energetic electrons

    Author(s): Horsfield A.P., Bowler D.R., Fisher A.J., Todorov T.N., Sanchez C.G.

    Journal Of Physics-Condensed Matter, 17, No. 30, pp. 4793-4812 (AUG 3 2005)

    doi: 10.1088/0953-8984/17/30/006

    Correlated electron-ion dynamics (CEID) is an extension of molecular dynamics that allows us to introduce in a correct manner the exchange of energy between electrons and ions. The formalism is based on a systematic approximation: small amplitude moment expansion. This formalism is extended here to include the explicit quantum spread of the ions and a generalization of the Hartree-Fock approximation for incoherent sums of Slater determinants. We demonstrate that the resultant dynamical equations reproduce analytically the selection rules for inelastic electron-phonon scattering from perturbation theory, which control the mutually driven excitations of the two interacting subsystems. We then use CEID to make direct numerical simulations of inelastic current-voltage spectroscopy in atomic wires, and to exhibit the crossover from ionic cooling to heating as a function of the relative degree of excitation of the electronic and ionic subsystems.

  23. Title: Size-dependent maximum in ion conductivity: The levitation effect provides an alternative explanation

    Author(s): Ghorai P.K., Yashonath S., Lynden-Bell R.M.

    Journal Of Physical Chemistry B, 109, No. 16, pp. 8120-8124 (APR 28 2005)

    doi: 10.1021/jp044605j

    We propose an alternative explanation of the size-dependent maximum in ion mobility in water in terms of the levitation effect, which accounts for the observed size-dependent maximum in the mobility of guest diffusion in porous media. In this explanation, the size at which the maximum occurs is related to the structure of the void space of the water; at the mobility maximum, the diffusant passes smoothly through necks connecting voids, and its potential energy shows minimum fluctuations. Molecular dynamics simulations of charged spheres of varying sizes are used to support this hypothesis. As in the levitation effect, the friction coefficient, the potential energy fluctuations, and the activation energy are found to be minima for particles with maximum self diffusivities similar to the guest diffusion in zeolites. Wavelength-dependent self diffusivities indicate a monotonic and oscillatory dependence, respectively, on wavenumber k for anomalous (AR) and linear regimes (LR). These are associated with single and biexponential decay of the incoherent intermediate scattering function.

  24. Title: Computational investigation of order, structure, and dynamics in modified water models

    Author(s): Lynden-Bell R.M., Debenedetti P.G.

    Journal Of Physical Chemistry B, 109, No. 14, pp. 6527-6534 (APR 14 2005)

    doi: 10.1021/jp0458553

    Model liquids have been constructed to study the role of local structure in the anomalous properties of liquid water. The intermolecular potentials were modified by increasing the weight of the Lennard-Jones term relative to the electrostatic term in the SPC/E model for water. The resulting family of liquids varies from SPC/E water to a Lennard-Jones-like liquid. Properties were measured as a function of density and temperature. The local structure was described by two order parameters, one measuring the tetrahedral order and the other measuring the translational order. The translational order parameter was found to be large for both tetrahedral and Lennard-Jones liquids, but to go through a minimum as the potentials were modified, demonstrating that the two types of structure are incompatible. Just as in water several properties (e.g., the translational diffusion coefficient, entropy) exhibit anomalous density dependence as a result of the breakdown of local tetrahedrality, we observed nonmonotonic behavior of the translational diffusion constant and reorientational relaxation rate as the fluids were transformed from tetrahedral to Lennard-Jones-like. This is also an indication of the incompatibility between Lennard-Jones and water-like structure.

  25. Title: The oxidation of NiAl: What can we learn from ab initio calculations?

    Author(s): Finnis M.W., Lozovoi A.Y., Alavi A.

    Annual Review Of Materials Research, 35, pp. 167-207 (2005)

    doi: 10.1146/annurev.matsci.35.101503.091652

    We review here the theory of the early stages of oxidation of the (110) surface of Ni1-xAlx, based on ab initio calculations using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces have been investigated, with oxygen coverages up to 2 ML of oxygen (I ML = 3 0 atoms per 2 surface Al atoms). The theory to date is a description in terms of equilibrium thermodynamics, with a comparison of the free energies of several surfaces of different composition, implemented at the atomic scale. Three environmental parameters are singled out as control variables in this treatment, namely the alloy composition x (assumed to be near 0.5), the temperature T and the partial pressure of oxygen p(O2), With certain reasonable approximations an analytic formula for the surface energy sigma is derived in terms of these variables and some constants that are calculated ab initio together with others that are derived from experimental thermodynamic tables. At oxygen pressures just above the threshold for bulk oxidation of NiAl, the calculations explain the observed formation of a thin film of alumina in place of NiAl surface layers, with the consequent dissolution of Ni into the bulk. Ab initio calculations illustrate how the energetics of supplying Al to the surface depends on bulk stoichiometry, which alters the relative stability of different surface oxidation states so as to favour oxidation more if the alloy is Al-rich than if it is Ni-rich.

  26. Title: Stability of Sr adatom model structures for SrTiO3(001) surface reconstructions

    Author(s): Liborio L.M., Sanchez C.G., Paxton A.T., Finnis M.W.

    Journal Of Physics-Condensed Matter, 17, No. 23, pp. L223-L230 (2005)

    doi: 10.1088/0953-8984/17/23/L01

    Full Text

    We report results of first-principles calculations on the thermodynamic stability of different Sr adatom structures that have been proposed to explain some of the observed reconstructions of the (001) surface of strontium titanate (Kubo and Nozoye 2003 Surf Sci. 542 177). From surface free energy calculations, a phase diagram is constructed indicating the range of conditions over which each structure is most stable. These results are compared with Kubo and Nozoye's experimental observations. It is concluded that low Sr adatom coverage structures can only be explained if the surface is far from equilibrium. Intermediate coverage structures are stable only if the surface is in or very nearly in equilibrium with the strontium oxide.

  27. Title: Modelling, simulations and monitoring of lamella structure formation in titanium alloys controlled by diffusion redistribution

    Author(s): Malinov S., Katzarov I., Sha W.

    Diffusion In Materials: Dimat 2004, Pt 1And 2, 237-240, pp. 635-646 (2005)


    A mathematical model and computer programs have been developed for simulation and monitoring 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 titanium alloys. Based on an integrated approach combining thermodynamic modelling and finite element simulation techniques, the a phase appearance at the grain boundary of beta phase is described by a numerical procedure for random nucleation as a function of the alloy composition 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 beta stabilising atoms ahead of the interface. This process is controlled by diffusion redistribution of the alloying elements between the alpha and beta phases that are different for different titanium alloys. The finite element method was used for solving the diffusion equation on the domain occupied by beta phase. Experimental data are used to build and validate the model. The models and program packages developed are used to simulate the morphology of the microstructure evolution in different titanium alloys during a variety of heat treatment procedures and can be used in practical processing routes.

  28. Title: Ring-opening and branching in polycarbonates: A density functional-Monte Carlo study

    Author(s): Jones R.O., Akola J., Ballone P.

    Advances In Polycarbonates, 898, pp. 200-213 (2005)


    Density functional calculations of the structures, potential energy surfaces, and reactivities for 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. The model comprises a fixed number of difunctional particles and harmonic bonds, and includes a low concentration (0.01% <= c(a) <= 0.36%) of monofunctional active particles able to modify pattern of the bonds without changing the total number. Monte Carlo simulations using this model show that in 2D and 3D there is a transition from unpolymerized cyclic oligomers at low density to a system of linear chains at high density. Entropy in the distribution of inter-particle bonds drives chain formation. The effects of branching defects are investigated by adding trifunctional units (of concentration c(3)). At sufficiently high density and c(3) values, the linking of polymer chains by trifunctional units gives rise to an aggregate (gel) incorporating most of the system mass.

  29. Title: Theory of the near K-edge structure in electron energy loss spectroscopy

    Author(s): Paxton A.T.

    Journal Of Electron Spectroscopy And Related Phenomena, 143, No. 2-3, pp. 51-64 (2005)

    doi: 10.1016/j.elspec.2004.05.007

    Full Text

    Arguments are given that lead to a formalism for calculating near K-edge structure in electron energy loss spectroscopy (EELS). This is essentially a one electron picture, while many body effects may be introduced at different levels, such as the local density approximation to density functional theory or the GW approximation to the electron self-energy. Calculations are made within the all electron LMTO scheme in crystals with complex atomic and electronic structures, and these are compared with experiment. (c) 2004 Elsevier B.V. All rights reserved.

  30. Title: Simulation of interfaces between room temperature ionic liquids and other liquids

    Author(s): Lynden-Bell R.M., Kohanoff J., Del Popolo M.G.

    Faraday Discussions, 129, pp. 57-67 (2005)

    doi: 10.1039/b405514d

    The structure and properties of the interfaces between the room temperature ionic liquid dimethylimidazolium chloride ([dmim]Cl) and different Lennard-Jones fluids and between ionic liquid and water have been studied by molecular dynamics simulations, and compared to the ionic liquid-vapour interface. Two contrasting types of interface were investigated, thermodynamically stable interfaces between ionic liquid and vapour and between ionic liquid and Lennard-Jones fluids, and diffusing interfaces between miscible phases of different compositions involving water. The density profiles of different species through the interface are presented. The cations and water molecules near the former type of interface are aligned relative to the surface, but no orientational preference was found near or in the broad diffusing interface. The ionic liquid has a negative electrostatic potential relative to vapour or Lennard-Jones fluid, but is more positive than pure water. This contrast is explained in terms of the relative importance of orientation and concentration differences in the two types of interface.