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  1. Title: Correlated electron-ion dynamics in metallic systems

    Author(s): Horsfield A.P., Finnis M., Foulkes M., LePage J., Mason D., Race C., Sutton A.P., Bowler D.R., Fisher A.J., Miranda R., Stella L., Stoneham A.M., Dundas D., McEniry E., Todorov T.N., Sanchez C.G.,

    Computational Materials Science, 44, No. 1, pp. 16-20 (November 2008)

    doi: 10.1016/j.commatsci.2008.01.055
    Abstract



    In this paper we briefly discuss the problem of simulating non-adiabatic processes in systems that are usefully modelled using molecular dynamics. In particular we address the problems associated with metals, and describe two methods that can be applied: the Ehrenfest approximation and correlated electron-ion dynamics (CEID). The Ehrenfest approximation is used to successfully describe the friction force experienced by an energetic particle passing through a crystal, but is unable to describe the heating of a wire by an electric current. CEID restores the proper heating. (C) 2008 Elsevier B.V. All rights reserved.

  2. Title: Negative heat capacities do occur. Comment on "Critical analysis of negative heat capacities in nanoclusters" by Michaelian K. and Santamaría-Holek I.

    Author(s): Lynden-Bell D., Lynden-Bell R.M.,

    Europhysics Letters, 82, No. 4, pp. 43001-43003 (MAY 9 2008)

    doi: 10.1209/0295-5075/82/43001
    Abstract



  3. Title: Ion Association in [bmim][PF6]/Naphthalene Mixtures: An Experimental and Computational Study

    Author(s): Del Popolo M.G., Mullan C., Holbrey J., Hardacre C., Ballone P.

    Journal of the American Chemical Society, 130, pp. 7032-7041 (May 2008)

    doi: 10.1021/ja710841n
    Abstract



    Mixtures of room temperature ionic liquids (IL) with neutral organic molecules provide a valuable testing ground to investigate the interplay of the ionic and molecular-dipolar state in dense Coulomb systems at near ambient conditions. In the present study, the viscosity η and the ionic conductivity σ of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])/naphthalene mixtures at T = 80 °C have been measured at 10 stoichiometries spanning the composition range from pure naphthalene to pure [bmim][PF6]. The viscosity grows nearly monotonically with increasing IL mole fraction (x), whereas the conductivity per ion displays a clear peak at x ≈ 15%. The origin of this maximum has been investigated using molecular dynamics simulations based on a classical force field. Snapshots of the simulated samples show that the conductivity maximum is due to the gradual transition in the IL component from an ionic state at high x to a dipolar fluid made of neutral ion pairs at low x. At concentrations x < 0.20 the ion pairs condense into molecular-thin filaments bound by dipolar forces and extending in between nanometric droplets of IL. These results are confirmed and complemented by the computation of dynamic and transport properties in [bmim][PF6]/naphthalene mixtures at low IL concentration.

  4. Title: A computational investigation of thermodynamics, structure, dynamics and solvation behavior in modified water models

    Author(s): Chaterjee S., Debenedetti P.G., Stillinger F.H., Lynden-Bell R.M.

    The Journal of Chemical Physics, 128, No. 12, pp. 124511-124520 (MAR 27 2008)

    doi: 10.1063/1.2841127
    Abstract



    We investigate the properties of geometrically modified water models by performing molecular dynamics simulations of perturbations of the extended simple point charge (SPC/E) model of water over a wide range of temperatures at 1 bar. The geometric modification consists of altering the H–O–H angle in SPC/E. The dipole moment is held constant by altering the O–H bond length, while the electrostatic charges are left unchanged. We find that a H–O–H angle of at least 100° is necessary for the appearance of density anomalies and of solubility extrema with respect to temperature for small apolar solutes. We observe the occurrence of two incompatible types of structural order in these models: Tetrahedral, with waterlike translational order for bent models with H–O–H angles in excess of 100°; and linear, with Lennard–Jones–like orientationally averaged translational order for smaller H–O–H angles. Increasing the H–O–H angle causes the density to increase, while at the same time shifting waterlike anomalies to progressively higher temperatures. For bent models with H–O–H angle greater than SPC/E's, we observe arrest of translational motion at 300 K (115°) and 330 K (120°).

  5. Title: Hydrophobic solvation of Gay–Berne particles in modified water models

    Author(s): Head-Gordon T., Lynden-Bell R.M.

    The Journal of Chemical Physics, 128, No. 10, pp. 104506-104513 (MAR 13 2008)

    doi: 10.1063/1.2837289
    Abstract



    The solvation of large hydrophobic solutes, modeled as repulsive and attractive Gay–Berne oblate ellipsoids, is characterized in several modified water liquids using the SPC/E model as the reference water fluid. We find that small amounts of attraction between the Gay–Berne particle and any model fluid result in wetting of the hydrophobic surface. However, significant differences are found among the modified and SPC/E water models and the critical distances in which they dewet the hydrophobic surfaces of pairs of repulsive Gay–Berne particles. We find that the dewetting trends for repulsive Gay–Berne particles in the various model liquids correlate directly with their surface tensions, the widths of the interfaces they form, and the openness of their network structure. The largest critical separations are found in liquids with the smallest surface tensions and the broadest interfaces as measured by the Egelstaff–Widom length.

  6. Title: First-principles molecular dynamics simulations of the interaction of ionic projectiles with liquid water and ice

    Author(s): Kohanoff J., Artacho E.

    5th International Conference on Radiation Damage in Biomolecular Systems (Debrecen, Hyngary, 2008), AIP Conference Proceedings, 1080, pp. 78-87 (JUN 13-15 2008)

    Abstract


    We first present results of first-principles molecular dynamics simulations of the passage of Carbon projectiles through water in the liquid state in the adiabatic regime, where the electrons are always in the instantaneous ground state. We study a range of projectile velocities up to the estimated upper limit for the adiabatic approximation and analyze the different types of collision events. We show that for high projectile velocities collisions are mostly binary, but at lower velocities most trajectories exhibit a continuous energy loss to the medium.. which cannot be properly described as a sequence of independent binary collisions. For the slowest projectiles we observe the formation of new chemical species such as hydronium, H5O2+ and hydrogen peroxide. When C-atoms are completely stopped, then we also see the formation of species like formic acid. By analyzing the generation of secondary fragments, we observe that these are mostly hyperthermal and their spatial rate of generation increases with decreasing projectile energy. The two most numerous species are H and OH. In the second part we study., via electronic dynamics with fixed nuclei, the opposite regime of very fast protons producing only electronic excitation in ice, under channeling conditions. We observe the existence of a threshold velocity for electronic excitation of about 0.2 a.u. By monitoring the rate of increase of the total energy, we calculate the electronic stopping power.

  7. Title: Nonlinear Relaxation in Redox Processes in Ionic and Polar Liquids

    Author(s): Streeter I., Lynden-Bell R.M., Compton R.G.,

    The Journal of Physical Chemistry C, 112, No. 37, pp. 14538-14544 (AUG 20 2008)

    doi: 10.1021/jp804958p
    Abstract



    Simulations have been performed to investigate the relaxation dynamics of the solvent around probe ions in the ionic liquid dimethylimidazolium hexafluorophosphate and in the polar liquid acetonitrile. The time scale of the relaxation dynamics is found to be different in the two cases, although our earlier work showed that the Marcus free energy curves, solvent rearrangement energies, and redox activation free energies were very similar. We also observe differences between the nonequilibrium decay curves S(t) and the time correlation functions C(t) for fluctuations at equilibrium showing that the response is strongly nonlinear. Relaxation toward equilibrium is slower for processes in which the magnitude of the electrostatic interaction increases than when it decreases. We discuss whether this may affect the rate constants for electrochemical processes and how this could be observed.

  8. Title: Redox potentials and screening in ionic liquids: Effects of sizes and shapes of solute ions

    Author(s): Lynden-Bell R.M.

    Journal of Chemical Physics, 129, No. 20, pp. 204503- (25 NOV 2008)

    doi: 10.1063/1.3020439
    Abstract



    Simulations of a model ionic liquid, [dmim][PF6] (dimethylimidazolium hexafluorophosphate), containing solute ions of different sizes and shapes have been used to investigate the changes in redox potentials of and screening around solute ions of different sizes and shapes. The effective solute size of spherical ions increases with the actual solute size although more slowly than expected. The effective solute size of tetrahedral or square planar ions varies little with actual ligand size. These results are clarified by reference to the charge density in the solvent around the ions, which is also used to calculate the potential within the solvent. Screening is essentially complete within 1 nm of the solute ion although charge density oscillations propagate further into the liquid. The results are compared to theoretical models and the implications for experiments are discussed.

  9. Title: Switching on magnetism in Ni-doped graphene: Density functional calculations

    Author(s): Santos E.J.G., Ayuela A., Fagan S.B., Mendes Filho J., Azevedo D.L., Souza Filho A.G., Sánchez-Portal D.

    Physical Review B, 78, No. 19, pp. 195420- (19 November 2008)

    doi: 10.1103/PhysRevB.78.195420
    Abstract



    Magnetic properties of graphenic carbon nanostructures, which are relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni (Nisub) impurities. Nisub defects are nonmagnetic in flat graphene and develop a nonzero-spin moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Nisub. A similar magnetic-nonmagnetic transition of Nisub can be expected by applying anisotropic strain to a flat graphene layer.

  10. Title: Melting behavior of an idealized membrane model

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

    Journal of Chemical Physics, 128, No. 2, pp. 024705-1-024705-10 (14 January 2008)

    doi: 10.1063/1.2804423
    Abstract



    The melting behavior of an idealized model giving rise to two-dimensional (2D) structures at low temperature and low density is investigated by Monte Carlo simulations. The system is made of particles carrying a spin of constant length and variable orientation, whose potential energy is the sum of a repulsive spherical pair interaction, and of a spin-spin contribution, reminiscent of but essentially different from the electrostatic dipole-dipole interaction. The simulation results show that the model phase diagram is determined by the interplay of a ferro- to paraelectric transition in the spin part and of the solid to fluid transition found in simple pair-potential models. The 2D solid melts into a three-dimensional (3D) fluid when the spin-spin interaction is weak. Strong spin-spin interactions give rise to two transitions, the first one corresponding to the melting of the 2D solid into a 2D fluid, and the second one corresponding to the crossover from a 2D to a 3D fluid. The fluid phase stable in between these two transitions provides a model for the liquid state arising in organic and biological membranes across their main transition.

  11. Title: Magnetic tight binding and the iron-chromium enthalpy anomaly

    Author(s): Paxton A.T., Finnis M.W.

    Physical Review B, 77, No. 2, Art. No. 024428 (2008)

    doi: 10.1103/PhysRevB.77.024428
    Abstract


    Full Text

    We describe a self consistent magnetic tight-binding theory based in an expansion of the Hohenberg-Kohn density functional to second order, about a non spin polarised reference density. We show how a first order expansion about a density having a trial input magnetic moment leads to a fixed moment model. We employ a simple set of tight-binding parameters that accurately describes electronic structure and energetics, and show these to be transferable between first row transition metals and their alloys. We make a number of calculations of the electronic structure of dilute Cr impurities in Fe which we compare with results using the local spin density approximation. The fixed moment model provides a powerful means for interpreting complex magnetic configurations in alloys; using this approach we are able to advance a simple and readily understood explanation for the observed anomaly in the enthalpy of mixing.

  12. Title: Boron in copper: A perfect misfit in the bulk and cohesion enhancer at a grain boundary

    Author(s): Lozovoi A.Y., Paxton A.T.

    Physical Review B, 77, No. 16, Art. No. 165413 (2008)

    doi: 10.1103/PhysRevB.77.165413
    Abstract


    Full Text

    Using first principles electronic structure methods, we calculate the effects of boron impurities in bulk copper and at surfaces and grain boundaries. We find that boron segregation to the Sigma5(310)[001] grain boundary should strengthen the boundary up to 1.5 ML coverage (15.24 at./nm2). The maximal effect is observed at 0.5 ML and corresponds to boron atoms filling exclusively grain boundary interstices. In copper bulk, B causes significant distortion both in interstitial and regular lattice sites, for which boron atoms are either too big or too small. The distortion is compensated to a large extent when the interstitial and substitutional boron combine together to form a strongly bound dumbbell. Our prediction is that bound boron impurities should appear in a sizable proportion if not dominate in most experimental conditions. A large discrepancy between calculated heats of solution and experimental terminal solubility of B in Cu is found, indicating either a significant failure of the density functional approach or, more likely, strongly overestimated solubility limits in the existing B-Cu phase diagram.

  13. Title: Electronic structure of the nitrogen-vacancy center in diamond from first-principles theory

    Author(s): Larsson J.A. Delaney P.

    Physical Review B, 77, Art. No. 165201 (2008)

    doi: 10.1103/PhysRevB.77.165201
    Abstract



    The nitrogen-vacancy (NV) center is a paramagnetic defect in diamond with applications as a qubit. Here, we investigate its electronic structure by using ab initio density functional theory for five different NV center models of two different cluster sizes. We describe the symmetry and energetics of the low-lying states and compare the optical frequencies obtained to experimental results. We compute the major transition of the negatively charged NV centers to within 25–100 meV accuracy and find that it is energetically favorable for substitutional nitrogens to donate an electron to NV0. The excited state of the major transition and the NV0 state with a neutral donor nitrogen are found to be close in energy.

  14. Title: Spontaneous spin polarization and electron localization in constrained geometries: The Wigner transition in nanowires

    Author(s): Hughes D., Ballone P.,

    Physical Review B, 77, No. 24, Art. No. 245312 (2008)

    doi: 10.1103/PhysRevB.77.245312
    Abstract



    The Wigner transition in a jellium model of cylindrical nanowires has been investigated by density-functional computations using the local spin-density approximation. A wide range of background densities ρb has been explored from the nearly ideal metallic regime (rs=[3/4πρb]1/3=1) to the high correlation limit (rs=100). Computations have been performed using an unconstrained plane wave expansion for the Kohn–Sham orbitals and a large simulation cell with up to 480 electrons. The electron and spin distributions retain the cylindrical symmetry of the Hamiltonian at high density, while electron localization and spin polarization arise nearly simultaneously in low-density wires (rs∼30). At sufficiently low density (rs≥40), the ground-state electron distribution is the superposition of well defined and nearly disjoint droplets, whose charge and spin densities integrate almost exactly to one electron and 1/2μB, respectively. Droplets are arranged on radial shells and define a distorted lattice whose structure is intermediate between bcc and fcc. Dislocations and grain boundaries are apparent in the droplets’ configuration found by our simulations. Our computations aim at modeling the behavior of experimental low-carried density systems made of lightly doped semiconductor nanostructures or conducting polymers.

  15. Title: Newtonian origin of the spin motive force in ferromagnetic atomic wires

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

    Physical Review B, 77, No. 5 (2008)

    doi: 10.1103/PhysRevB.77.054439
    Abstract



    We demonstrate numerically the existence of a spin-motive force acting on spin carriers when moving in a time and space dependent internal field. This is the case for electrons in a one-dimensional wire with a precessing domain wall. The effect can be explained solely by adiabatic dynamics and is shown to exist for both classical and quantum systems.

  16. Title: Interatomic potential for the Cu-Ta system and its application to surface wetting and dewetting

    Author(s): Hashibon A., Lozovoi A.Y., Mishin Y., Elsasser C., Gumbsch P.,

    Physical Review B, 77, No. 9 (2008)

    doi: 10.1103/PhysRevB.77.094131
    Abstract



    An angle- dependent interatomic potential has been developed for the Cu- Ta system by crossing two existing potentials for pure Cu and Ta. The cross- interaction functions have been fitted to first- principles data generated in this work. The potential has been extensively tested against first- principles energies not included in the fitting database and applied to molecular dynamics simulations of wetting and dewetting of Cu on Ta. We find that a Cu film placed on a Ta ( 110 ) surface dewets from it, forming a Cu droplet on top of a stable Cu monolayer. We also observe that a drop of liquid Cu placed on a clean Ta ( 110 ) surface spreads over it as a stable monolayer, while the extra Cu atoms remain in the drop. The stability of a Cu monolayer and instability of thicker Cu films are consistent with recent experiments and first- principles calculations. This agreement demonstrates the utility of the potential for atomistic simulations of Cu- Ta interfaces.

  17. Title: Compton scatter profiles for warm dense matter

    Author(s): Sahoo S., Gribakin G.F., Naz G.S., Kohanoff J., Riley D.,

    Physical Review E, 77, No. 4 (2008)

    doi: 10.1103/PhysRevE.77.046402
    Abstract



    In this paper, we discuss the possibility of using x-ray Compton scattering as a probe of the outer electronic structure of ions immersed in warm dense matter. It is proposed that the x-ray free-electron lasers currently under construction will provide an ideal tool for this, with the main pulse being used to create a uniform well-defined sample and the third harmonic providing a clean monochromatic probe. We model the plasma photon scatter spectrum by combining self-consistent finite-temperature electronic structure calculations with molecular dynamics simulations of the ion-ion structure factor. In particular, we present bound-free Compton profiles that are more accurate that those obtained using form factor or impulse approximations.

  18. Title: The importance of solvent reorganisation in the effect of an ionic liquid on a unimolecular substitution process

    Author(s): Yau H.M., Barnes S.A., Hook J.M., Youngs T.G.A., Croft A.K., Harper J.B.,

    Chemical Communications, No. 30, pp. 3576-3578 (2008)

    doi: 10.1039/b805255g
    Abstract



    Temperature dependent rate studies demonstrate an enthalpic bene. t and an entropic cost associated with the change in the rate of a unimolecular substitution process on addition of a high proportion of an ionic liquid, the latter e. ect being supported by molecular dynamics simulations.

  19. Title: Inelastic quantum transport in nanostructures: The self-consistent Born approximation and correlated electron-ion dynamics

    Author(s): McEniry E.J., Frederiksen T., Todorov T.N., Dundas D., Horsfield A.P.,

    Physical Review B, 78, No. 3 (2008)

    doi: 10.1103/PhysRevB.78.035446
    Abstract



    A dynamical method for inelastic transport simulations in nanostructures is compared to a steady-state method based on nonequilibrium Green's functions. A simplified form of the dynamical method produces, in the steady state in the weak-coupling limit, effective self-energies analogous to those in the Born approximation due to electron-phonon coupling. The two methods are then compared numerically on a resonant system consisting of a linear trimer weakly embedded between metal electrodes. This system exhibits an enhanced heating at high biases and long phonon equilibration times. Despite the differences in their formulation, the static and dynamical methods capture local current-induced heating and inelastic corrections to the current with good agreement over a wide range of conditions, except in the limit of very high vibrational excitations where differences begin to emerge.

  20. Title: Effect of quantization of vibrations on the structural properties of crystals

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

    Physical Review B, 78, No. 22 (2008)

    doi: 10.1103/PhysRevB.78.224108
    Abstract



    We study the structural effects produced by the quantization of vibrational degrees of freedom in periodic crystals at zero temperature. To this end we introduce a methodology based on mapping a suitable subspace of the vibrational manifold and solving the Schrodinger equation in it. A number of increasingly accurate approximations ranging from the quasiharmonic approximation (QHA) to the vibrational self-consistent field (VSCF) method and the exact solution are described. A thorough analysis of the approximations is presented for model monatomic and hydrogen-bonded chains, and results are presented for a linear H-F chain where the potential-energy surface is obtained via first-principles electronic structure calculations. We focus on quantum nuclear effects on the lattice constant and show that the VSCF is an excellent approximation, meaning that correlation between modes is not extremely important. The QHA is excellent for covalently bonded mildly anharmonic systems, but it fails for hydrogen-bonded ones. In the latter, the zero-point energy exhibits a nonanalytic behavior at the lattice constant where the H atoms center, which leads to a spurious secondary minimum in the quantum-corrected energy curve. An inexpensive anharmonic approximation of noninteracting modes appears to produce rather good results for hydrogen-bonded chains for small system sizes. However, it converges to the incorrect QHA results for increasing size. Isotope effects are studied for the first-principles H-F chain. We show how the lattice constant and the H-F distance increase with decreasing mass and how the QHA proves to be insufficient to reproduce this behavior.

  21. Title: Evidence of short-range screening in shock-compressed aluminum plasma

    Author(s): Saiz E.G., Gregori G., Khattak F.Y., Kohanoff J., Sahoo S., Naz G.S., Bandyopadhyay S., Notley M., Weber R.L., Riley D.,

    Physical Review Letters, 101, No. 7 (2008)

    doi: 10.1103/PhysRevLett.101.075003
    Abstract



    We have investigated the angular variation in elastic x-ray scattering from a dense, laser-shock-compressed aluminum foil. A comparison of the experiment with simulations using an embedded atom potential in a molecular dynamics simulation shows a significantly better agreement than simulations based on an unscreened one-component plasma model. These data illustrate, experimentally, the importance of screening for the dense plasma static structure factor.

  22. Title: Comment on “Huge Excitonic Effects in Layered Hexagonal Boron Nitride”

    Author(s): Wirtz L., Marini A., Gruening M., Attaccalite C., Kresse G., Rubio A.,

    Physical Review Letters, 100, pp. 189701- (2008)

    Abstract


  23. Title: On thin ice: surface order and disorder during pre-melting

    Author(s): Bishop C.L., Pan G., Liu L.M., Tribello G., . Michaelides A., Wang E., Slater B.

    Faraday Discussions, 141, pp. 277-292 (2008)

    Abstract


  24. Title: Surface Energy and surface proton order of ice Ih

    Author(s): Pan D., Liu L.-M., Tribello G., Slater B., Michaelides A., E. Wang E.

    Phys. Rev. Lett., 101, pp. 155703- (2008)

    Abstract


  25. Title: Quantum chessboards in the deuterium molecular ion

    Author(s): Calvert C.R., Birkeland T., King R.B., Williams I.D., McCann J.F.

    Journal of Physics B: Atomic, Molecular and Optical Physics, 41, pp. 205504- (2008)

    doi: 10.1088/0953-4075/41/20/205504
    Abstract



    We present a new algorithm for vibrational control in deuterium molecules that is feasible with current experimental technology. A pump mechanism is used for creating a coherent superposition of the D+2 vibrations. A short, intense infrared control pulse is applied after a chosen delay time to create selective interferences. A 'chessboard' pattern of states can be realized in which a set of even- or odd-numbered vibrational states can be selectively annihilated or enhanced. A technique is proposed for experimental realization and observation of this effect using 5 fs pulses of λ = 790 nm radiation, with intermediate intensity (5 × 1013 W cm−2).