Recent Publications

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  1. Title: Magnetism and thermodynamics of defect-free Fe-Cr alloys

    Author(s): Klaver T.P.C., Drautz R., Finnis M.W.

    Physical Review B, 74, No. 9, Art. No. 094435 (SEP 2006)

    doi: 10.1103/PhysRevB.74.094435
    Abstract



    Density functional theory calculations have been used to study the mixing behavior of Fe-Cr alloys. The heats of formation Delta E-f of 65 Fe-Cr structures in their magnetic ground states have been determined. A positive Delta E-f is found over most of the concentration range. From 0-12 % Cr a small negative Delta E-f down to -8 meV/atom is found. The origin of the negative Delta E-f in Fe-rich structures is traced to the solution energy of single Cr atoms. At low concentration, Cr atoms in Fe repel each other, causing ordering. The Cr-Cr interactions are well reproduced even without the self-consistent relaxation of the electron density and the positions of atoms. Multi-ion (or concentration-dependent) interactions are indispensable in order to describe the whole phase diagram. The interesting magnetic situation that arises when ferromagnetic and antiferromagnetic metals are mixed in different ratios is discussed with reference to nearest- and next-nearest-neighbor clusters of Cr in Fe. Magnetic frustration leads to a strong dependence of the Cr moment on the number of Cr neighbors. The "normal" chemical-mixing energy and the influence of magnetism are distinguished by comparing magnetic and nonmagnetic calculations for similar systems.

  2. Title: Using DL_POLY to study the sensitivity of liquid structure to potential parameters

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

    Molecular Simulation, 32, No. 12-13, pp. 1025-1033 (OCT-NOV 2006)

    doi: 10.1080/08927020600823133
    Abstract



    Two case studies are presented showing the local structure in liquids and how it responds to changes in the intermolecular potential. The idea is to use realistic and unrealistic potentials in order to determine the sensitivity of local liquid structure to potential parameters. The first case study concerns two families of modified water models. In the "hybrid" family, the hydrogen bond strength is reduced, but the geometry kept constant; in the second family, the molecular geometry is changed by reducing the bond angle, keeping a constant molecular dipole moment. The local structure is measured by radial distribution functions, three-dimensional probability distribution functions and three-body angular correlations. The second case study concerns the ionic liquid dimethylimidazolium chloride ([C(1)mim]Cl). The effect of reducing the hydrogen bonding potential of the cations while maintaining their charge is examined.

  3. Title: Solvation in modified water models: towards understanding hydrophobic effects

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

    Molecular Physics, 104, No. 22-24, pp. 3593-3605 (NOV-DEC 2006)

    doi: 10.1080/00268970601022727
    Abstract



    The solvation of small hydrophobic solutes, modelled as hard spheres or Lennard-Jones particles, is characterized in several modified water liquids. In the hybrid family of liquids the SPC/E model is partially transformed to a Lennard-Jones liquid with the same number density. In this family, hydrophobic solutes become less soluble as the liquid structure becomes more close packed. In the bent family of models the network structure is altered by geometrical changes from SPC/E which increase the solubility of hydrophobic groups. Solvophobicity in the isotropic model, which has the same radial distribution function as SPC/E water by construction, is greater than in SPC/E water. This shows the importance of three-body correlations. In addition to excess chemical potentials, contact densities and the agreement with the Gaussian distributions predicted by information theory are investigated. Dielectric constants and surface tensions have been determined approximately and are used in the discussion of the results.

  4. Title: The surface structure of ionic liquids: Comparing simulations with x-ray measurements

    Author(s): Sloutskin E., Lynden-Bell R.M., Balasubramanian S., Deutsch M.

    Journal Of Chemical Physics, 125, No. 17, Art. No. 174715 (NOV 7 2006)

    doi: 10.1063/1.2361289
    Abstract



    The surface-normal electron density profile of an ionic liquid, [bmim][PF6], derived from x-ray reflectivity measurements, is compared with two independent molecular-dynamics simulations. It is shown that a meaningful comparison requires a detailed accounting for both thermal and nonthermal surface roughening effects. The former is due to thermally excited capillary waves, and the latter is due to the molecular zero-point motion and form. These quantities influence very significantly, but differently, the simulated and measured density profiles. Stripping off these effects from both types of profiles yields the intrinsic structure factor of the surface. The simulated intrinsic structure factors are found to deviate considerably from the measured one. The introduction of additional ad hoc surface roughness to the simulated profiles greatly reduces the deviation, however, no physical origin for this effect can be identified. The method employed in this study should prove useful for simulation-experiment comparisons of other liquid surfaces, provided they obey capillary-wave theory, as do almost all liquid surfaces studied to date by x-ray reflectivity. (c) 2006 American Institute of Physics.

  5. Title: A molecular dynamics study of glucose solvation in the ionic liquid 1,3-dimethylimidazolium chloride

    Author(s): Youngs T.G.A., Holbrey J.D., Deetlefs M., Nieuwenhuyzen M., Gomes M.F.C., Hardacre C.

    Chemphyschem, 7, No. 11, pp. 2279-2281 (NOV 13 2006)

    doi: 10.1002/cphc.200600569
    Abstract



  6. Title: Angular-dependent interatomic potential for tantalum

    Author(s): Mishin Y., Lozovoi A.Y.

    Acta Materialia, 54, No. 19, pp. 5013-5026 (NOV 2006)

    doi: 10.1016/j.actamat.2006.06.034
    Abstract



    A new angular-dependent semi-empirical interatomic potential suitable for atomistic simulations of plastic deformation, fracture and related processes in body-centered cubic Ta has been constructed by fitting to experimental properties and a first-principles database generated in this work. The potomial reasonably reproduces a variety of properties of Ta, including elastic constants, thermal expansion, high-pressure behavior, the vacancy formation and migration energies, surface energies, gamma surfaces on the [110] and 12 111 planes, energy along the twinning and anti-twinning deformation paths, structure of the [211] twin boundary and energies of alternate crystal structures of Ta. The potential s applied to calculate the core structure of the 1/2(111) screw dislocation and the critical resolved shear stress as a function of the angli, between the (211) slip plane and the maximum-stress plane. The results are in good agreement with previous first-principles calcula:ions and experimentally known mechanical behavior of body-centered cubic metals. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  7. Title: Solvation structure and transport of acidic protons in ionic liquids: A first-principles simulation study

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

    Journal Of Physical Chemistry B, 110, No. 17, pp. 8798-8803 (MAY 4 2006)

    doi: 10.1021/jp0602326
    Abstract



    Ab initio simulations of a single molecule of HCl in liquid dimethyl imidazolium chloride [dmim][Cl] show that the acidic proton exists as a symmetric, linear ClHCl- species. Details of the solvation structure around this molecule are given. The proton-transfer process was investigated by applying a force along the antisymmetric stretch coordinate until the molecule broke. Changes in the free energy and local solvation structure during this process were investigated. In the reaction mechanism identified, a free chloride approaches the proton from the side. As the original ClHCl- distorts and the incoming chloride forms a new bond to the proton, one of the original chlorine atoms is expelled and a new linear molecule is formed.

  8. Title: Development of complex classical force fields through force matching to ab initio data: Application to a room-temperature ionic liquid

    Author(s): Youngs T.G.A., Del Popolo M.G., Kohanoff J.

    Journal Of Physical Chemistry B, 110, No. 11, pp. 5697-5707 (MAR 23 2006)

    doi: 10.1021/jp056931k
    Abstract



    Recent experimental neutron diffraction data and ab initio molecular dynamics simulation of the ionic liquid dimethylimidazolium chloride ([dmim]Cl) have provided a structural description of the system at the molecular level. However, partial radial distribution functions calculated front the latter, when compared to previous classical Simulation results, highlight some limitations in the structural description offered by force field-based simulations. With the availability of ab initio data it is possible to improve the classical description of [dmim]Cl by using the force matching approach, and the strategy for fitting complex force fields in their original functional form is discussed. A self-consistent optimization rnethod for the generation of classical potentials of general functional forill is presented and applied, and a force field that better reproduces the observed first principles forces is obtained. When used in Simulation, it predicts structural data which reproduces more faithfully that observed in the ab initio studies. Some possible refinements to the technique, its application, and the general Suitability of common potential energy functions used within many ionic liquid force fields are discussed.

  9. Title: Density functional theory study of tetrathiafulvalene and thianthrene in acetonitrile: Structure, dynamics, and redox properties

    Author(s): VandeVondele J., Lynden-Bell R., Meijer E.J., Sprik M.

    Journal Of Physical Chemistry B, 110, No. 8, pp. 3614-3623 (MAR 2 2006)

    doi: 10.1021/jp054841+
    Abstract



    The redox potentials of the organic compounds tetrathiafulvalene (TTF) and thianthrene (TH) in an explicit aprotic polar solvent, acetonitrile, have been computed using ab initio molecular dynamics simulation based on a Gaussian basis set methodology. The density functional description of the pure solvent yields a diffuse and mobile liquid, with structural and dynamical properties that are in good agreement with earlier classical models and experiment. Molecular dynamics simulation of both solute species in their neutral and radical cation states combined with free energy difference calculations result in estimates for the redox potentials of the reactions TH center dot+ + TTF -> TH + TTF center dot+ and TH2+ + TTF center dot+ -> TH center dot+ + TTF2+. The obtained values are 0.95 +/- 0.06 and 1.09 +/- 0.06 V, respectively, in excellent agreement with experimental data of 0.93 and 1.08 V. Our computational approach is based on Marcus theory, assuming quadratic free energy surfaces. We show that this approximation can still be accurate in systems, such as TH, that undergo a significant change in geometry upon oxidation. Furthermore, despite the different localization of the spin density in the radical cations, results based on self-interaction-corrected functionals and on standard generalized gradient approximations are identical to within 10 meV.

  10. Title: Simple models of complex aggregation: Vesicle formation by soft repulsive spheres with dipolelike interactions

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

    Physical Review E, 73, No. 3, Art. No. 031404 (MAR 2006)

    doi: 10.1103/PhysRevE.73.031404
    Abstract



    Structural and thermodynamic properties of spherical particles carrying classical spins are investigated by Monte Carlo simulations. The potential energy is the sum of short range, purely repulsive pair contributions, and spin-spin interactions. These last are of the dipole-dipole form, with however, a crucial change of sign. At low density and high temperature the system is a homogeneous fluid of weakly interacting particles and short range spin correlations. With decreasing temperature particles condense into an equilibrium population of free floating vesicles. The comparison with the electrostatic case, giving rise to predominantly one-dimensional aggregates under similar conditions, is discussed. In both cases condensation is a continuous transformation, provided the isotropic part of the interatomic potential is purely repulsive. At low temperature the model allows us to investigate thermal and mechanical properties of membranes. At intermediate temperatures it provides a simple model to investigate equilibrium polymerization in a system giving rise to predominantly two-dimensional aggregates.

  11. Title: Magnetomechanical interplay in spin-polarized point contacts

    Author(s): Stamenova M., Sahoo S., Sanchez C.G., Todorov T.N., Sanvito S.

    Physical Review B, 73, No. 9, Art. No. 094439 (MAR 2006)

    doi: 10.1103/PhysRevB.73.094439
    Abstract



    We investigate the interplay between magnetic and structural dynamics in ferromagnetic atomic point contacts. In particular, we look at the effect of the atomic relaxation on the energy barrier for magnetic domain wall migration and, reversely, at the effect of the magnetic state on the mechanical forces and structural relaxation. We observe changes of the barrier height due to the atomic relaxation up to 200%, suggesting a very strong coupling between the structural and the magnetic degrees of freedom. The reverse interplay is weak; i.e., the magnetic state has little effect on the structural relaxation at equilibrium or under nonequilibrium, current-carrying conditions.

  12. Title: Molecular conduction: Do time-dependent simulations tell you more than the Landauer approach?

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

    Journal Of Chemical Physics, 124, No. 21, Art. No. 214708 (JUN 7 2006)

    doi: 10.1063/1.2202329
    Abstract



    A dynamical method for simulating steady-state conduction in atomic and molecular wires is presented which is both computationally and conceptually simple. The method is tested by calculating the current-voltage spectrum of a simple diatomic molecular junction, for which the static Landauer approach produces multiple steady-state solutions. The dynamical method quantitatively reproduces the static results and provides information on the stability of the different solutions. (c) 2006 American Institute of Physics.

  13. Title: Independent particle descriptions of tunneling using the many-body quantum transport approach

    Author(s): Fagas G., Delaney P., Greer J.C.

    Physical Review B, 73, No. 24, Art. No. 241314 (JUN 2006)

    doi: 10.1103/PhysRevB.73.241314
    Abstract



    Currents across thin insulators are commonly taken as single electrons moving across classically forbidden regions; this independent particle picture is well-known to describe most tunneling phenomena. Examining quantum transport from a different perspective, i.e., by explicit treatment of electron-electron interactions, we evaluate different single particle approximations with specific application to tunneling in metal-molecule-metal junctions. We find maximizing the overlap of a Slater determinant composed of single-particle states to the many-body current-carrying state is more important than energy minimization for defining single-particle approximations in a system with open boundary conditions. Thus the most suitable single particle effective potential is not one commonly in use by electronic structure methods, such as the Hartree-Fock or Kohn-Sham approximations.

  14. Title: Electronic structure calculations for solids and molecules: theory and computational methods

    Author(s): Kohanoff J.

    Cambridge University Press (JUN 2006)

    Abstract

    Full Text

    Electronic structure problems are studied in condensed matter physics and theoretical chemistry to provide important insights into the properties of matter. This graduate textbook describes the main theoretical approaches and computational techniques, from the simplest approximations to the most sophisticated methods. It starts with a detailed description of the various theoretical approaches to calculating the electronic structure of solids and molecules, including density-functional theory and chemical methods based on Hartree-Fock theory. The basic approximations are thoroughly discussed, and an in-depth overview of recent advances and alternative approaches in DFT is given. The second part discusses the different practical methods used to solve the electronic structure problem computationally, for both DFT and Hartree-Fock approaches. Adopting a unique and open approach, this textbook is aimed at graduate students in physics and chemistry, and is intended to improve communication between these communities. It also serves as a reference for researchers entering the field.

  15. Title: Optimization through quantum annealing: theory and some applications

    Author(s): Battaglia D.A., Stella L.,

    Contemporary Physics, 47, No. 4, pp. 195-208 (July-August 2006)

    doi: 10.1080/00107510600861454
    Abstract



    Quantum annealing is a promising tool for solving optimization problems, similar in some ways to the traditional ( classical) simulated annealing of Kirkpatrick et al. Simulated annealing takes advantage of thermal fluctuations in order to explore the optimization landscape of the problem at hand, whereas quantum annealing employs quantum fluctuations. Intriguingly, quantum annealing has been proved to be more effective than its classical counterpart in many applications. We illustrate the theory and the practical implementation of both classical and quantum annealing - highlighting the crucial differences between these two methods - by means of results recently obtained in experiments, in simple toy-models, and more challenging combinatorial optimization problems ( namely, Random Ising model and Travelling Salesman Problem). The techniques used to implement quantum and classical annealing are either deterministic evolutions, for the simplest models, or Monte Carlo approaches, for harder optimization tasks. We discuss the pro and cons of these approaches and their possible connections to the landscape of the problem addressed.

  16. Title: Dissociative electron attachment to HCCCN

    Author(s): Graupner K., Merrigan T.L., Field T.A., Youngs T.G.A., Marr P.C.

    New Journal Of Physics, 8, Art. No. 117 (JUL 20 2006)

    doi: 10.1088/1367-2630/8/7/117
    Abstract



    Dissociative electron attachment to cyanoacetylene (propiolonitrile) HCCCN has been observed in the electron energy range 0-12 eV. Negative ions are formed in two main bands with maxima at similar to 1.6 eV (CCCN-) and similar to 5.3 eV (CCCN-, CN-, HCC- and CC-). There are also weaker resonances which lead to dissociative electron attachment to form CN-, HCC- and CC- with a maximum intensity at similar to 8.1 eV and CCCN-, CN- and CC- at similar to 11.2 eV. A trace of CCN- is observed at similar to 9.1 eV. The positions of the main dissociative attachment bands observed are close to positions of pi* resonances recently calculated by Sommerfeld and Knecht. Calculations have also been performed in this work, which confirm the position of the p* orbitals. The electron affinity of the CCCN radical is determined as 4.59 +/- 0.25 eV from the threshold for CCCN- formation at 1.32 +/- 0.15 eV. Dissociative electron attachment to this molecule will act as a source of negative ions in extraterrestrial environments where electrons are present with more than 1.3 eV energy.

  17. Title: Classical computation with quantum systems

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

    Proceedings Of The Royal Society A-Mathematical Physical And Engineering Sciences, 462, No. 2065, pp. 117-135 (JAN 8 2006)

    doi: 10.1098/rspa.2005.1565
    Abstract



    As semiconductor electronic devices scale to the nanometer range and quantum structures (molecules, fullerenes, quantum dots, nanotubes) are investigated for use in information processing and storage, it, becomes useful to explore the limits imposed by quantum mechanics on classical computing. To formulate the problem of a quantum mechanical description of classical computing, electronic device and logic gates are described as quantum sub-systems with inputs treated as boundary conditions, outputs expressed.is operator expectation values, and transfer characteristics and logic operations expressed through the sub-system Hamiltonian. with constraints appropriate to the boundary conditions. This approach, naturally, leads to a description of the subsystem.,, in terms of density matrices. Application of the maximum entropy principle subject to the boundary conditions (inputs) allows for the determination of the density matrix (logic operation), and for calculation of expectation values of operators over a finite region (outputs). The method allows for in analysis of the static properties of quantum sub-systems.

  18. Title: A phase-field model for computer simulation of lamellar structure formation in gamma-TiAl

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

    Acta Materialia, 54, No. 2, pp. 453-463 (JAN 2006)

    doi: 10.1016/j.actamat.2005.09.016
    Abstract



    In the present paper, a phase-field model is developed to simulate the formation and evolution of lamellar microstructure in gamma-TiAl alloys. The mechanism of formation of TiAl lamellae proposed by Denquin and Naka is incorporated into the model. The model describes the formation and evolution of the face-centered cubic (fcc) stacking lamellar zone followed by the subsequent appearance and growth of the gamma-phase, involving both the chemical composition change by atom transfer and the ordering of the fee lattice. The thermodynamics of the model system and the interaction between the displacive and diffusional transformations are described by a non-equilibrium free energy formulated as a function of concentration and structural order parameter fields. The long-range elastic interactions, arising from the lattice misfit between the alpha, fcc (A1) and the various orientation variants of the gamma-phase are taken into account by incorporating of the elastic strain energy into the total free energy. Simulation studies based on the model successfully predicted some essential features of the lamellar structure. It is found that the formation and evolution of the lamellar structure are predominantly controlled by the minimization of the elastic energy of the interfaces between the different fcc stacking groups, low-symmetry product phase gamma and the high-symmetry alpha-phase, as well as between the various orientation variants of the product phase. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  19. Title: Modelling of kinetics of nitriding titanium alloys

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

    Surface Engineering, 22, No. 6, pp. 452-454 (DEC 2006)

    doi: 10.1179/174327806X124717
    Abstract



    Models for simulation and monitoring of the evolution of surface layers during gas nitriding of titanium alloys were developed using own experimental results. They are based on analytical and numerical solutions of the diffusion equation and model the nitrogen distribution, the thickness of the nitrided layers and the incubation time for the formation of layers on the surface of titanium alloys.

  20. Title: Molecular electrostatic properties of ions in an ionic liquid

    Author(s): Prado C.E.R., Del Popolo M.G., Youngs T.G.A., Kohanoff J., Lynden-Bell R.M.

    Molecular Physics, 104, No. 15, pp. 2477-2483 (AUG 10 2006)

    doi: 10.1080/00268970600765748
    Abstract



    We have analysed the electronic wave functions from an ab initio simulation of the ionic liquid ( room temperature molten salt) dimethyl imidazolium chloride ([dmim][Cl] or [C(1)mim][CI]) using localized Wannier orbitals. This allows us to assign electron density to individual ions. The probability distributions of the ionic dipole moments for an isolated ion and for ions in solution are compared. The liquid environment is found to polarize the cation by about 0.7D and to increase the amplitude of the fluctuations in the dipole moments of both cation and anion. The relative changes in nuclear and electronic contributions are shown. The implications for classical force fields are discussed.

  21. Title: New bis(triazinyl) pyridines for selective extraction of americium(III)

    Author(s): Hudson M.J., Boucher C.E., Braekers D., Desreux J.F., Drew M.G.B., Foreman M.R.S., Harwood L.M., Hill C., Madic C., Marken F., Youngs T.G.A.

    New Journal Of Chemistry, 30, No. 8, pp. 1171-1183 (AUG 2006)

    doi: 10.1039/b514108g
    Abstract



    New hydrophobic, tridentate nitrogen heterocyclic reagents (BATPs) such as 2,6-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenzo[1,2,4]triazin-3-yl) pyridine (1) and 2,6-bis(9,9,10,10-tetramethyl-9,10-dihydro-1,2,4-triaza-anthrane-3-yl) pyridine (2) have been studied. I is resistant to hydrolysis in 3 M nitric acid, whereas 2 is resistant to both acid hydrolysis and radiolysis. The molecules are able to give significantly enhanced separations of americium(III) from an excess of europium(III) in nitric acid. Typically, for 1 D-Am = 500 and SFAm,/Eu = 5000 compared with D-Am = 30 and SFAm /Eu = 400 with the reference molecule 2,6-bis(isopropyl[1,2,4]triazin-3-yl) pyridine (7). In order to increase the stability of 1 and 2, the labile alpha-benzylic hydrogens that are present in 7 have been replaced by alkyl groups. Three molecules of 1 are able to enclose completely the coordination sphere of the M(III) in the crystal structure of [Y(1)(3)][Y(NO3)(5)]center dot NO3 center dot 2.5H(2)O.

  22. Title: Monte Carlo studies of quantum and classical annealing on a double well

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

    Physical Review B, 73, No. 14, Art. No. 144302 (April 2006)

    doi: 10.1103/PhysRevB.73.144302
    Abstract



    We present results for a variety of Monte Carlo annealing approaches, both classical and quantum, benchmarked against one another for the textbook optimization exercise of a simple one-dimensional double well. In classical (thermal) annealing, the dependence upon the move chosen in a Metropolis scheme is studied and correlated with the spectrum of the associated Markov transition matrix. In quantum annealing, the path integral Monte Carlo approach is found to yield nontrivial sampling difficulties associated with the tunneling between the two wells. The choice of fictitious quantum kinetic energy is also addressed. We find that a "relativistic" kinetic energy form, leading to a higher probability of long real-space jumps, can be considerably more effective than the standard nonrelativistic one.

  23. Title: The transfer of energy between electrons and ions in solids

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

    Reports On Progress In Physics, 69, No. 4, pp. 1195-1234 (APR 2006)

    doi: 10.1088/0034-4885/69/4/R05
    Abstract



    In this review we consider those processes in condensed matter that involve the irreversible flow of energy between electrons and nuclei that follows from a system being taken out of equilibrium. We survey some of the more important experimental phenomena associated with these processes, followed by a number of theoretical techniques for studying them. The techniques considered are those that can be applied to systems containing many nonequivalent atoms. They include both perturbative approaches (Fermi's Golden Rule and non-equilibrium Green's functions) and molecular dynamics based (the Ehrenfest approximation, surface hopping, semi-classical Gaussian wavefunction methods and correlated electron-ion dynamics). These methods are described and characterized, with indications of their relative merits.

  24. Title: Coupling between lattice vibrations and magnetism in Ising-like systems

    Author(s): Triguero C., Porta M., Planes A.

    Physical Review B, 73, No. 5, pp. 054401- (1 February 2006)

    doi: 10.1103/PhysRevB.73.054401
    Abstract



    In this paper the bond proportion model is introduced as a prototype of a system with coupled magnetic and vibrational degrees of freedom. This model is generalized within the framework of cluster expansions in order to achieve invariance of the potential energy to a rotation of the crystal. First, the original bond proportion model is solved in the mean-field approximation and by means of numerical simulations. It has been found that the temperature and the smoothness of the magnetic phase transition depend on the strength of the magnetoelastic coupling. For a large enough entropy difference between the magnetic phases the phase transition becomes first order. This is evidenced by means of the computation of the magnetization, the elastic constants, and the total entropy. The numerical simulation of the modified bond proportion model has revealed significant differences with respect to the bond proportion model in the heat capacity around the phase transition and, consequently, in the entropy difference between the magnetic phases. Small differences in the elastic constants are also detected.

  25. Title: Structural and chemical embrittlement of grain boundaries by impurities: A general theory and first-principles calculations for copper

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

    Physical Review B, 74, No. 15, Art. No. 155416 (2006)

    doi: 10.1103/physRevB.74.155416
    Abstract


    Full Text

    First-principles calculations of the Sigma 5(310)[001] symmetric tilt grain boundary in Cu with Bi, Na, and Ag substitutional impurities provide evidence that in the phenomenon of Bi embrittlement of Cu grain boundaries electronic effects do not play a major role; on the contrary, the embrittlement is mostly a structural or "size" effect. Na is predicted to be nearly as good an embrittler as Bi, whereas Ag does not embrittle the boundary in agreement with experiment. While we reject the prevailing view that "electronic" effects (i.e., charge transfer) are responsible for embrittlement, we do not exclude the role of chemistry. However, numerical results show a striking equivalence between the alkali metal Na and the semimetal Bi, small differences being accounted for by their contrasting "size" and "softness" (defined here). In order to separate structural and chemical effects unambiguously if not uniquely, we model the embrittlement process by taking the system of grain boundary and free surfaces through a sequence of precisely defined gedanken processes; each of these representing a putative mechanism. We thereby identify three mechanisms of embrittlement by substitutional impurities, two of which survive in the case of embrittlement or cohesion enhancement by interstitials. Two of the three are purely structural and the third contains both structural and chemical elements that by their very nature cannot be further unraveled. We are able to take the systems we study through each of these stages by explicit computer simulations and assess the contribution of each to the net reduction in intergranular cohesion. The conclusion we reach is that embrittlement by both Bi and Na is almost exclusively structural in origin; that is, the embrittlement is a size effect.

  26. Title: Simulation of the surface structure of butylmethylimidazolium ionic liquids

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

    Physical Chemistry Chemical Physics, 8, No. 8, pp. 949-954 (2006)

    doi: 10.1039/b514848k
    Abstract



    Molecular dynamics simulations of the liquid/vacuum surfaces of the room temperature ionic liquids [bmim][PF6], [bmim][BF4] and [bmim][Cl] have been carried out at various temperatures. The surfaces are structured with a top monolayer containing oriented cations and anions. The butyl side chains tend to face the vacuum and the methyl side chains the liquid. However, as the butyl chains are not densely packed, both anions and rings are visible from the vacuum phase. The effects of temperature and the anion on the degree of cation orientation is small, but the potential drop from the vacuum to the interior of the liquid is greater for liquids with smaller anions. We compare the simulation results with a range of experimental observations and suggest that neutron reflection from samples with protiated butyl groups would be a sensitive probe of the structure.

  27. Title: Density functionals from many-body perturbation theory: The band gap for semiconductors and insulators

    Author(s): Gruening M., Marini A., Rubio A.

    Journal of Chemical Physics, 124, pp. 15408- (2006)

    Abstract


  28. Title: Effect of spatial nonlocality on the density functional band gap

    Author(s): Gruening M., Marini A., Rubio A.,

    Physical Reviews B, 74, pp. 161103- (2006)

    Abstract


  29. Title: A blind structure prediction of ice XIV

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

    J. Am. Chem. Soc., 128, pp. 12594-12595 (2006)

    Abstract


  30. Title: Proton ordering energetics in ice phases

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

    Chem. Phys. Lett. , 425, pp. 246-250 (2006)

    Abstract