Recent Publications

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  1. Title: Melting of a tetrahedral network model of silica

    Author(s): Cabriolu R., Del Popolo M.G., Ballone P.

    Physical Chemistry Chemical Physics, 11, pp. 10820-10823 (SEP 25 2009)

    doi: 10.1039/b913358e
    Abstract



    Thermal properties of an idealised tetrahedral network model of silica are investigated by Monte Carlo simulations. The interatomic potential consists of anharmonic stretching and bending terms, plus a short range repulsion. The model includes a bond interchange rule similar to the well known Wooten, Winer and Weaire (WWW) algorithm (see Phys. Rev. Lett., 1985, 54, 1392). Simulations reveal an apparent first order melting transition at T = 2200 K. The computed changes in the local coordination upon melting are consistent with experimental and ab-initio data.

  2. Title: Nanometric ionic-liquid films on silica: a joint experimental and computational study

    Author(s): Bovio S., Podesta S., Milani P., Ballone P., Del Popolo M.G.

    Journal of Physics C: Condensed Matter , 21, pp. 424118-1 -424118-10 (OCT 5 2009)

    doi: 10.1088/0953-8984/21/42/424118
    Abstract



    Atomic force microscopy images for [bmim][Tf2N] films deposited at ambient conditions by drop-casting show a population of terraced islands of mesoscopic area (1–100 μ2) and ∼50 nm height. The regularity of terraces and steps, stiff mechanical properties and a fragile fracture mode all suggest that the islands are solid-like, even though bulk [bmim][Tf2N] is liquid at the temperature of the experiment. Molecular dynamics simulations for a homogeneous [bmim][Tf2N] film 4 nm thick on silica also display marked layering in proximity to silica of periodicity closely matching the experimental estimate of the step height. The density modulation of the simulated sample, however, decays into an approximatively homogeneous and fluid-like density distribution ∼2 nm from the solid surface. The detailed comparison of experiments and simulations is contained in the closing section of the paper.

  3. Title: Simulations of imidazolium ionic liquids: when does the cation charge distribution matter?

    Author(s): Lynden-Bell R.M., Youngs T.G.A.,

    Journal of Physics: Condensed Matter, 21, No. 42, Art. No. 424120 (OCT 21 2009)

    doi: 10.1088/0953-8984/21/42/424120
    Abstract



    We compare the properties of models of liquids and crystals constructed from a number of intermolecular potentials for dimethylimidazolium chloride [dmim][Cl]. The force fields differ in the charge distribution in the cation but all include short range interactions which determine the shape of the cation. In addition to 'realistic' models intended for simulation of [dmim][Cl] we take two extreme 'unrealistic' models in which the cation charge is localized on the ring atoms or at the ring centre in order to study the effects of the cation charge distribution. The effects of polarizability are investigated by using shell models for the chloride ion. We find that, while equilibrium properties such as energetics, crystal structure, liquid structure and charge screening depend on the charge distribution in the cation but are little affected by including polarizability, dynamical properties such as diffusion are strongly affected by polarizability.

  4. Title: Amphiphilic Character and Aggregation Properties of Small Cholesterol Islands on Water: A Simulation Study

    Author(s): Cromie S.R.T., Del Popolo M.G., Ballone P.

    Journal Of Physical Chemistry B, 113, No. 14, pp. 4674-4687 (MAR 10 2009)

    doi: 10.1021/jp8084759
    Abstract



    Small cholesterol clusters (Chn, 1 ≤ n ≤ 10) on water have been investigated by molecular dynamics simulations based on an empirical force field potential. The simulation results for clusters of increasing size highlight the processes that take place during the initial stages of cholesterol aggregation at low coverage. Our results show that at T = 280 K clusters form spontaneously out of a dilute two-dimensional (2D) vapor of cholesterol molecules, driven by entropy and potential energy. Up to n = 10, corresponding to 25% coverage for our simulation cell, cholesterol molecules lay flat on the water surface, forming fluid-like 2D aggregates. Within each island, the elongated cholesterol molecules align their longest axis along a common direction, anticipating the liquid-crystal character of bulk phases. With increasing cluster size, the adsorption energy per molecule quickly saturates to a value close to the limiting value for a full monolayer coverage. Cholesterol adsorption locally changes the electrostatic surface polarization of water, giving rise to an induced moment that tends to compensate the dipole of the adsorbed island. Computations for a single cholesterol molecule and dimer in bulk water are reported for a comparison. The absorption energy of both species in bulk water is larger than their adsorption energy at the water surface, thus pointing to entropy as the origin of the amphiphilic character of cholesterol.

  5. Title: An introduction to the tight binding approximation---implementation by diagonalisation

    Author(s): Paxton A.T.

    Winter School: Multiscale Simulation Methods in Molecular Sciences, Forschungszentrum Jülich (MAR 2009)

    Abstract

    Full Text

  6. Title: General local and rectilinear vibrational coordinates consistent with Eckart’s conditions

    Author(s): Scivetti I., Kohanoff J., Gidopoulos N.,

    Physical Review A, 79, No. 3, Art. No. 032516 (MAR 2009)

    doi: 10.1103/PhysRevA.79.032516
    Abstract



    We present a general method to construct a set of local rectilinear vibrational coordinates for a nonlinear molecule whose reference structure does not necessarily correspond to a stationary point of the potential-energy surface. We show both analytically and with a numerical example that the vibrational coordinates satisfy Eckart’s conditions. In addition, we find that the Watson Hamiltonian provides a fairly robust description even of highly excited vibrational states of triatomic molecules, except for a few states of large amplitude motion sampling the singular region of the Hamiltonian. These states can be identified through slow convergence.

  7. Title: Ion Association in 1-Butyl-3-methylimidazolium Hexafluorophosphate / Naphthalene Mixtures

    Author(s): Del Popolo M.G., Ballone P.

    ACS Symposium Series, 1030, pp. 403-418 (DEC 18 2009)

    doi: 10.1021/bk-2009-1030.ch026
    Abstract



    Mixtures of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) and naphthalene have been investigated by molecular dynamics simulations based on an empirical potential model, covering the full range of concentrations from the pure ionic liquid to pure naphthalene. The composition dependence of thermodynamic functions and of dynamical properties highlights a continuous but relatively sharp transition taking place at 20% ionic liquid concentration. The analysis of the electric conductivity and of simulation snapshots reveals that the transition marks the transformation of the [C4mim][PF6] bonding from molecular-dipolar at low concentration to ionic at high concentration. The probability distribution for the microscopic electric fields acting on the naphthalene atoms broadens significantly in going from the molecular to the ionic state, suggesting that optical spectroscopy and core level spectroscopy could be used to probe the transition at the atomistic scale.

  8. Title: Interaction of Room Temperature Ionic Liquid Solutions with a Cholesterol Bilayer

    Author(s): Cromie S., Del Popolo M.G., Ballone P.

    Journal Of Physical Chemistry B, 113, No. 34, pp. 11642-11648 (AUG 5 2009)

    doi: 10.1021/jp904060y
    Abstract



    Water solutions of representative ([C4mim][Cl] and [C4mim][Tf2N]) room temperature ionic liquids (ILs) in contact with a neutral lipid bilayer made of cholesterol molecules has been investigated by molecular dynamics simulations based on an empirical force field model. The results show that both ILs display selective adsorption at the water−cholesterol interface, with partial inclusion of ions into the bilayer. In the case of [C4mim][Cl], the adsorption of ions at the water−cholesterol interface is limited by a sizable bulk solubility of the IL, driven by the high water affinity of [Cl]−. The relatively low solubility of [C4mim][Tf2N], instead, gives rise to a nearly complete segregation of the IL component on the bilayer, altering its volume, compressibility, and electrostatic environment. The computational results display important similarities to the results of recent experimental measurements for ILs in contact with phospholipid model membranes (see Evans, K. O. Int. J. Mol. Sci. 2008, 9, 498−511 and references therein).

  9. Title: Ring currents in azulene

    Author(s): Paxton A.T., Todorov T.N., Elena A.M.

    Chemical Physics Letters, 483, No. 1-3, pp. 154-158 (2009)

    doi: 10.1016/j.cplett.2009.10.041
    Abstract


    Full Text

    We propose a self consistent polarisable ion tight binding theory for the study of push–pull processes in aromatic molecules. We find that the method quantitatively reproduces ab initio calculations of dipole moments and polarisability. We apply the scheme in a simulation which solves the time dependent Schrödinger equation to follow the relaxation of azulene from the second excited to the ground states. We observe rather spectacular oscillating ring currents which we explain in terms of interference between the HOMO and LUMO states.

  10. Title: Atomistic studies of <101] screw dislocation core structures and glide in γ-TiAl

    Author(s): Katzarov I.H., Paxton A.T.

    Philosophical Magazine, 89, No. 21, pp. 1731-1750 (2009)

    doi: 10.1080/14786430903037281
    Abstract


    Full Text

    The core structure of superdislocations in L10 TiAl was investigated with a view to clarifying their dissociation abilities and the mechanisms by which they may become sessile by self-locking. A detailed knowledge of the fine structure of dislocations is essential in analysing the origin of the various deformation features. Atomistic simulation of the core structure and glide of the screw superdislocation was carried out using a bond order potential for γ-TiAl. The core structure of the screw superdislocation was examined, starting with initial unrelaxed configurations corresponding to various dislocation dissociations discussed in the literature. The superdislocation was found to possess in the screw orientation either planar (glissile) or non-planar (sessile) core structures. The response of the core configurations to externally applied shear stress was studied. Some implications were considered of the dissociated configurations and their response to externally applied stress on dislocation dynamics, including the issue of dislocation decomposition, the mechanism of locking and the orientation dependence of the dislocation substructure observed in single-phase γ-TiAl. An unexpectedly rich and complex set of candidate core structures, both planar and non-planar, was found, the cores of which may transform under applied stress with consequent violation of Schmid's law.

  11. Title: Atomistic studies of interactions between the dominant lattice dislocations and γ/γ-lamellar boundaries in lamellar γ-TiAl

    Author(s): Katzarov I.H., Paxton A.T.

    Acta Materialia, 57, No. 11, pp. 3349-3366 (2009)

    doi: 10.1016/j.actamat.2009.03.042
    Abstract


    Full Text

    This paper reports on atomistic simulations of the interactions between the dominant lattice dislocations in γ-TiAl (ordinary screw 1/2<101] and <101] superdislocations) with all three kinds of γ/γ-lamellar boundaries in polysynthetically twinned (PST) TiAl. The purpose of this study is to clarify the early stage of lamellar boundary controlled plastic deformation in PST TiAl. The interatomic inter- actions in our simulations are described by a bond order potential for L10-TiAl which provides a proper quantum mechanical description of the bonding. We are interested in the dislocation core geometries that the lattice produces in proximity to lamellar boundaries and the way in which these cores are affected by the elastic and atomistic effects of dislocation-lamellar boundary interaction. We study the way in which the interfaces affect the activation of ordinary dislocation and superdislocation slip inside the γ-lamellae and transfer of plastic deformation across lamellar boundaries. We find three new phenomena in the atomic-scale plasticity of PST TiAl, particularly due to elastic and atomic mismatch associated with the 60° and 120° γ/γ-interfaces: (i) two new roles of the γ/γ-interfaces, i.e. decomposition of superdislocations within 120° and 60° interfaces and subsequent detachment of a single ordinary dislocation and (ii) blocking of ordin- ary dislocations by 60° and 120° interfaces resulting in the emission of a twinning dislocation.

  12. Title: Current-driven atomic waterwheels

    Author(s): Dundas D., McEniry E.J., Todorov T.N.

    Nature Nanotechnology, 4, No. 2, pp. 99-102 (2009)

    doi: 10.1038/nnano.2008.411
    Abstract



    A current induces forces on atoms inside the conductor that carries it. It is now possible to compute these forces from scratch, and to perform dynamical simulations of the atomic motion under current. One reason for this interest is that current can be a destructive force—it can cause atoms to migrate, resulting in damage and in the eventual failure of the conductor. But one can also ask, can current be made to do useful work on atoms? In particular, can an atomic-scale motor be driven by electrical current, as it can be by other mechanisms? For this to be possible, the current-induced forces on a suitable rotor must be non-conservative, so that net work can be done per revolution. Here we show that current-induced forces in atomic wires are not conservative and that they can be used, in principle, to drive an atomic-scale waterwheel.

  13. Title: Supercell size scaling of density functional theory formation energies of charged defects

    Author(s): Hine N.D.M., Frensch K., Foulkes W.M.C., Finnis M.W.,

    Physical Review B, 79, No. 2 (2009)

    doi: 10.1103/PhysRevB.79.024112
    Abstract



    We address the calculation within density functional theory (DFT) of defect formation energies in alumina, a ceramic oxide often considered an archetype for a wide variety of other similar oxides. We examine the conditions under which calculated defect formation energies, especially those of charged defects, are independent of the principal approximations of the plane-wave DFT formalism, most significant of which is the finite-sized supercell in which the calculation must be performed. We introduce a variation on existing methods of extrapolation to infinite system size to reduce dependence of the result on finite-size errors in the electrostatic and elastic energies of a periodic supercell containing a defect. We also show how the results can be made relatively insensitive to the choice of exchange-correlation functional and pseudopotential by a suitable treatment of the chemical potentials of the atomic species. Our results for formation energies of charged defects are less sensitive than traditional approaches to supercell size and choices of exchange-correlation functional and pseudopotential, and differ notably from previous results.

  14. Title: Current-assisted cooling in atomic wires

    Author(s): McEniry E.J., Todorov T.N., Dundas D.,

    Journal of Physics: Condensed Matter, 21, No. 19 (2009)

    doi: 10.1088/0953-8984/21/19/195304
    Abstract



    The effects of inelastic interactions between current-carrying electrons and vibrational modes of a nanoscale junction are a major limiting factor on the stability of such devices. A method for dynamical simulation of inelastic electron-ion interactions in nanoscale conductors is applied to a model system consisting of an adatom bonded to an atomic wire. It is found that the vibrational energy of such a system may decrease under bias, and furthermore that, as the bias is increased, the rate of cooling, within certain limits, will increase. This phenomenon can be understood qualitatively through low-order perturbation theory, and is due to the presence of an anti-resonance in the transmission function of the system at the Fermi level. Such current-assisted cooling may act as a stabilization mechanism, and may form the basis for a nanoscale cooling 'fan'.

  15. Title: Vacancy segregation in the initial oxidation stages of the TiN(100) surface

    Author(s): Zimmermann J., Finnis M.W., Ciacchi L.C.,

    Journal of Chemical Physics, 130, No. 13 (2009)

    doi: 10.1063/1.3105992
    Abstract



    The well-known corrosion resistance and biocompatibility of TiN depend on the structural and chemical properties of the stable oxide film that forms spontaneously on its surface after exposure to air. In the present work, we focus on the atomistic structure and stability of the TiN(100) surface in contact with an oxidizing atmosphere. The early oxidation stages of TiN(100) are investigated by means of first-principles molecular dynamics (FPMD). We observe selective oxidation of Ti atoms and formation of an ultrathin Ti oxide layer, while Ti vacancies are left behind at the metal/oxide interface. Within the formalism of ab initio thermodynamics we compute the segregation energies of vacancies and vacancy clusters at the metal/oxide interface, comparing the stability of the system obtained by FPMD simulations with ideally reconstructed models. We find that the localization of Ti vacancies in the thin oxide layer and at the TiN/oxide interface is thermodynamically stable and may account for the early removal of N atoms from the interface by segregation of N vacancies from the bulk reservoir. We suggest that superficial oxidation may proceed along two distinct possible pathways: a thermodynamically stable path along the potential energy minimum surface and a metastable, kinetically driven path that results from the high heat release during the dissociation of O-2.

  16. Title: Vacancy segregation in the initial oxidation stages of the TiN(100) surface.

    Author(s): Zimmermann J., Finnis M.W., Ciacchi L.C.,

    Journal of Chemical Physics, 130, No. 13, pp. 134714- (2009)

    Abstract


    The well-known corrosion resistance and biocompatibility of TiN depend on the structural and chemical properties of the stable oxide film that forms spontaneously on its surface after exposure to air. In the present work, we focus on the atomistic structure and stability of the TiN(100) surface in contact with an oxidizing atmosphere. The early oxidation stages of TiN(100) are investigated by means of first-principles molecular dynamics (FPMD). We observe selective oxidation of Ti atoms and formation of an ultrathin Ti oxide layer, while Ti vacancies are left behind at the metal/oxide interface. Within the formalism of ab initio thermodynamics we compute the segregation energies of vacancies and vacancy clusters at the metal/oxide interface, comparing the stability of the system obtained by FPMD simulations with ideally reconstructed models. We find that the localization of Ti vacancies in the thin oxide layer and at the TiN/oxide interface is thermodynamically stable and may account for the early removal of N atoms from the interface by segregation of N vacancies from the bulk reservoir. We suggest that superficial oxidation may proceed along two distinct possible pathways: a thermodynamically stable path along the potential energy minimum surface and a metastable, kinetically driven path that results from the high heat release during the dissociation of O(2).

  17. Title: Evaluation of Gas Solubility Prediction in Ionic Liquids using COSMOthermX

    Author(s): Ab Manan N., Hardacre C., Jacquemin J., Rooney D.W., Youngs T.G.A.,

    Journal of Chemical and Engineering Data, 54, No. 7, pp. 2005-2022 (2009)

    doi: 10.1021/je800857x
    Abstract



    As the range of available ionic liquids increases, methods by which important engineering parameters such as gas solubilities can be estimated from simple structural information become ever more desirable. COSMO-based thermodynamic models, such as that used by COSMOthermX, allow the determination of such data for pure and mixed component systems. Herein, we evaluate the predictive capability of COSMOthermX through a comparison with literature data obtained from the IUPAC database which contains data for 15 gases in 27 ionic liquids, To determine any effect inherent to ionic liquids, gas solubility predictions were first performed for selected molecular solvents at constant temperature and pressure. Further estimations of gas solubility at temperatures ranging from (278 to 368) K at 0.1 MPa in water were performed for 14 gases. The Study has demonstrated that COSMOthermX is capable of predicting, qualitatively, gas solubilities in ionic liquids and, hence, reducing the amount of unnecessary experimental measurements prior to specific applications using ionic liquids.

  18. Title: Structural motifs of cholesterol nanoparticles

    Author(s): Cromie S.R.T., Ballone P.,

    Journal of Chemical Physics, 131, No. 3 (2009)

    doi: 10.1063/1.3179683
    Abstract



    The growth sequence of gas-phase cholesterol clusters (Ch(N)) with up to N=36 molecules has been investigated by atomistic simulation based on an empirical force field model. The results of long annealings from high temperature show that the geometric motifs characterizing the structure of pure cholesterol crystals already appear in nanometric aggregates. In all clusters molecules tend to align along a common direction. For cluster sizes above the smallest ones, dispersion interactions among the hydrocarbon body and tails of cholesterol cooperate with hydrogen bonding to give rise to a bilayer structure. Analysis of snapshots from the annealing shows that the condensation of hydrogen bonds into a connected network of rings and chains is an important step in the self-organization of cholesterol clusters. The effect of solvation on the equilibrium properties of medium-size aggregates is investigated by short molecular dynamics simulations for the N=30 and N=40 clusters in water at near ambient conditions and in supercritical carbon dioxide at T=400 K.

  19. Title: Self-consistent geometry in the computation of the vibrational spectra of molecules

    Author(s): Scivetti I., Kohanoff J., Gidopoulos N.,

    Physical Review A, 80, No. 2 (2009)

    doi: 10.1103/PhysRevA.80.022516
    Abstract



    An exact and general approach to study molecular vibrations is provided by the Watson Hamiltonian. Within this framework, it is customary to omit the contribution of the terms with the vibrational angular momentum and the Watson term, especially for the study of large systems. We discover that this omission leads to results which depend on the choice of the reference structure. The self-consistent solution proposed here yields a geometry that coincides with the quantum averaged geometry of the Watson Hamiltonian and appears to be a promising way for the computation of the vibrational spectra of strongly anharmonic systems.

  20. Title: A shell model for the H-bonded ferroelectric KH2PO4

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

    Physica B: Condensed Matter, 404, No. 18, pp. 2736-2738 (2009)

    doi: 10.1016/j.physb.2009.06.143
    Abstract



    A shell model for KH2PO4 (KDP), the prototype compound of the family of H-bonded ferroelectric materials, has been constructed by adjusting the interaction parameters to first-principles calculations. Structural properties, energy barriers, phonons, and the relative stability between the ferroelectric (FE) phase and a relevant antiferroelectric metastable structure associated to domain walls, compare very favorably to available first-principles and experimental data. Molecular dynamics simulations show that the model behaves satisfactorily within the FE phase. This model will be used to study the elusive structure of the paraelectric (PE) phase and the nature of the FE-PE phase transition. (C) 2009 Elsevier B.V. All rights reserved.

  21. Title: Energetics of charged point defects in rutile TiO2 by density functional theory

    Author(s): Li X., Finnis M.W., He J., Behera R.K., Phillpot S.R., Sinnott S.B., Dickey E.C.,

    Acta Materialia, 57, No. 19, pp. 5882-5891 (2009)

    doi: 10.1016/j.actamat.2009.08.014
    Abstract



    The defect formation energies of all possible charge states of point defects in TiO2, including titanium interstitials, titanium vacancies and oxygen vacancies, are calculated in the phase space of temperature, oxygen partial pressure and Fermi level by combining density functional theory (DFT) and thermodynamic calculations. The point defect phase diagram illustrates that fully charged defects dominate in most regimes. The calculations not only give reasonable defect formation energies compared with prior experimental measurements, but also predict n-type TiO2 at high T and low P-O2, and p-type TiO2 at low T and high P-O2, which agrees well with experimental data. In addition, we evaluate methods for correcting the effects of artificial electrostatic interactions caused by periodic boundary conditions in the DFT calculations, including the electrostatic potential alignment correction (Delta V correction) and the Makov-Payne correction. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  22. Title: Mesophases in Nearly 2D Room-Temperature Ionic Liquids

    Author(s): Manini N., Cesaratto M., Del Popolo M.G., Ballone P.,

    Journal of Physical Chemistry B, 113, No. 47, pp. 15602-15609 (2009)

    doi: 10.1021/jp907924z
    Abstract



    Computer simulations of (i) a [C(12)mim][Tf2N] film of nanometric thickness squeezed at kbar pressure by a piecewise parabolic confining potential reveal a mesoscopic in-plane density and composition modulation reminiscent of mesophases seen in 3D samples of the same room-temperature ionic liquid (RTIL). Near 2D confinement, enforced by a high normal load, as well as relatively long aliphatic chains are strictly required for the mesophase formation, as confirmed by computations for two related systems made of (ii) the same [C(12)mim][Tf2N] adsorbed at a neutral solid surface and (iii) a shorter-chain RTIL ([C(4)mim][Tf2N]) trapped in the potential well of part i. No in-plane modulation is seen for ii and iii. In case ii, the optimal arrangement of charge and neutral tails is achieved by layering parallel to the surface, while, in case iii, weaker dispersion and packing interactions are unable to bring aliphatic tails together into mesoscopic islands, against overwhelming entropy and Coulomb forces. The onset of in-plane mesophases could greatly affect the properties of long-chain RTILs used as lubricants.

  23. Title: Structure of warm dense matter via angularly resolved x-ray scatter

    Author(s): Riley D., Gareta J.J.A., Saiz E.G., Khattak F.Y., Kohanoff J., Sahoo S., Naz G.S., Shearer S.F.C., Thornton K.A., Gregory C., Woolsey N.C., Notley M., Neely D.,

    Plasma Physics and Controlled Fusion, 51, No. 12 (2009)

    doi: 10.1088/0741-3335/51/12/124036
    Abstract



    In this paper we describe experimental results on angularly resolved x-ray scatter from a sample of warm dense aluminium that has been created by double sided laser-driven shock compression. The experiment was carried out on the Central Laser Facility of the Rutherford Appleton Laboratory, using the VULCAN laser. The form of the angularly resolved scatter cross-section was compared with predictions based on a series of molecular dynamics simulations with an embedded atom potential, a Yukakwa potential and a bare Coulomb potential. The importance of screening is evident from the comparison and the embedded atom model seems to match experiment better than the Yukawa potential.

  24. Title: Liquid Structure and Dynamics of Aqueous Isopropanol over gamma-Alumina

    Author(s): Youngs T.G.A., Weber D., Gladden L.F., Hardacre C.,

    Journal of Physical Chemistry C, 113, No. 51, pp. 21342-21352 (2009)

    doi: 10.1021/jp906677c
    Abstract



    The liquid structures of thin films of aqueous solutions of 0, 7, 19, 50, and 100 mol % isopropanol above O/Al-terminated gamma-alumina < 001 > surfaces have been investigated by means of classical molecular dynamics simulations. The structuring effect of the oxide oil the liquid mixtures is strong and heavily dependent on the local structure of the oxide. Two distinct re-ions are found oil the oxide Surface characterized by the degree of coordination of Al atoms. Above octahedral Al atoms, water and isopropanol molecules adsorb via the oxygen atoms to maximize the electrostatic interaction, whereas above tetrahedral Al sites the solvent molecules adsorb via hydrogen atoms with the oxygen atoms away front the surface. More mobility is found in the second layer compared with the first; however, its structure is still influenced significantly by the orientation of molecules in the first adsorbed layer. Qualitatively, the displacement of water from the surface by the adsorption of isopropanol occurs with 2.6 Water molecules lost for every alcohol molecule present based on the effective surface areas of the two species calculated from the pure simulations. Diffusion in the liquid has been investigated by both molecular dynamics and PFG-NMR Studies. Both show that the first adsorbed layer is slower moving than the bulk by several orders of magnitude, as expected, and thereafter the simulations show a gradual increase in diffusivity with increasing distance from the interface, lending toward the bulk value. Experimental diffusion coefficients of isopropanol inside the gamma-alumina pore are found to be approximately one-quarter of that found in the bulk liquid. When compared with the simulated Values, this suggests that the Surface properties probed by the NMR technique encompass the first two layers interacting with the surface. In addition, because of the time scale of the measurements, the largest diffusion coefficient obtained includes the pore tortuosity and, therefore, a reduced experimental value is found compared with that obtained front the simulations.

  25. Title: Spontaneous spin polarization in geometrically constricted metal nanowires

    Author(s): Cortes-Huerto R., Ballone P.,

    Physical Review B, 80, No. 23 (2009)

    doi: 10.1103/PhysRevB.80.233304
    Abstract



    The electronic structure of thin conducting wires with a narrow geometric constriction has been determined by density-functional theory computations in the local spin density approximation. Spontaneous spin polarization arises in nominally paramagnetic wires at sufficiently low density (r(s)>= 15). Real-space spin-polarization maps show a fascinating variety of magnetic structures pinned at the constriction. The frequency-dependent conductivity is different for the spin-up and spin-down channels and significantly lower than in wires of identically vanishing spin polarization.

  26. Title: Electronic properties of zircon and hafnon from many-body perturbation theory

    Author(s): Shaltaf R., Rangel T., Gruening M., Gonze X., Rignanese G., Hamann D.R.,

    Physical Review B, 79, pp. 195101 - (2009)

    Abstract


  27. Title: Yambo: An ab initio tool for excited state calculations

    Author(s): Marini A., Hogan C., Gruening M., Varsano D.

    Comp. Phys. Comm., 180, pp. 1392- (2009)

    Abstract


  28. Title: Exciton-plasmon states in nanoscale materials: breakdown of the Tamm-Dancoff approximation

    Author(s): Gruening M., Marini A., Gonze X.

    Nano Letters, 9, pp. 2820-2824 (2009)

    Abstract


  29. Title: A molecular dynamics study of the early states of calcium carbonate growth

    Author(s): Tribello G., Bruneval F., Liew C.C., Parrinello M.

    J. Phys. Chem. B,, 113, pp. 11680-11687 (2009)

    Abstract


  30. Title: The phase diagram of water at negative pressures: virtual ices

    Author(s): Conde M., Vega C., Tribello G., Slater B.

    J. Chem. Phys., 131, pp. 034510- (2009)

    Abstract


  31. Title: A theoretical examination of known and hypothetical clathrate hydrate stability

    Author(s): Tribello G., Slater B.

    J. Chem. Phys. , 131, pp. 024703- (2009)

    Abstract


  32. Title: Binding of calcium and carbonate to polyacrylates

    Author(s): Tribello G., Liew C., Parrinello M.

    J. Phys. Chem. B. , 113, pp. 7081-7085 (2009)

    Abstract


  33. Title: Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

    Author(s): Hummelshøj J.S., Landis D.D., Voss J., Jiang T., Tekin A., Bork N., Dułak M., Mortensen J.J., Adamska L., Andersin J., Baran J.D., Barmparis G.D., Bell F., Bezanilla A.L., Bjork J., Björketun M.E., Bleken F., Buchter F., Bürkle M., Burton P.D., Buus B.B., Calborean A., Calle-Vallejo F., Casolo S., Chandler B.D., Chi D.H., Czekaj I, Datta S., Datye A., DeLaRiva A., Despoja V, Dobrin S., Engelund M., Ferrighi L., Frondelius P., Fu Q., Fuentes A., Fürst J., García-Fuente A., Gavnholt J., Goeke R., Gudmundsdottir S., Hammond K.D., Hansen H.A., Hibbitts D., Hobi E., Howalt J.G., Hruby S.L., Huth A., Isaeva L., Jelic J., Jensen I.J.T., Kacprzak K.A., Kelkkanen A., Kelsey D., Kesanakurthi D.S., Kleis J., Klüpfel P.J., Konstantinov I, Korytar R., Koskinen P., Krishna C., Kunkes E., Larsen A.H., Lastra J.M.G., Lin H., Lopez-Acevedo O., Mantega M., Martínez J.I., Mesa I.N., Mowbray D.J., Mýrdal J.S.G., Natanzon Y., Nistor A., Olsen T., Park H., Pedroza L.S., Petzold V, Plaisance C., Rasmussen J.A., Ren H., Rizzi M., Ronco A.S., Rostgaard C., Saadi S., Salguero L.A., Santos E.J.G., Schoenhalz A.L., Shen J., Smedemand M., Stausholm-Møller O.J., Stibius M., Strange M., Su H.B., Temel B., Toftelund A., Tripkovic V, Vanin M., Viswanathan V, Vojvodic A., Wang S., Wellendorff J., Thygesen K.S., Rossmeisl J., Bligaard T., Jacobsen K.W., Nørskov J.K., Vegge T.

    The Journal of Chemical Physics, 131, pp. 014101- (2009)

    doi: 10.1063/1.3148892
    Abstract



    We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M1); and 1 alkali, alkaline earth or 3d/4dtransition metal atom (M2) plus two to five (BH4)− groups, i.e., M1M2(BH4)2–5, using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M1(Al/Mn/Fe)(BH4)4, (Li/Na)Zn(BH4)3, and (Na/K)(Ni/Co)(BH4)3 alloys are found to be the most promising, followed by selected M1(Nb/Rh)(BH4)4 alloys