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ABSTRACT: The structure of liquid water at ambient conditions is studied in ab initio molecular dynamics simulations in the NVE ensemble using van der Waals (vdW) density-functional theory, i.e., using the new exchange-correlation functionals optPBE-vdW and vdW-DF2, where the latter has softer nonlocal correlation terms. Inclusion of the more isotropic vdW interactions counteracts highly directional hydrogen bonds, which are enhanced by standard functionals. This brings about a softening of the microscopic structure of water, as seen from the broadening of angular distribution functions and, in particular, from the much lower and broader first peak in the oxygen-oxygen pair-correlation function (PCF) and loss of structure in the outer hydration shells. Inclusion of vdW interactions is shown to shift the balance of resulting structures from open tetrahedral to more close-packed. The resulting O-O PCF shows some resemblance with experiment for high-density water (Soper, A. K. and Ricci, M. A. Phys. Rev. Lett. 2000, 84, 2881), but not directly with experiment for ambient water. Considering the accuracy of the new functionals for interaction energies, we investigate whether the simulation protocol could cause the deviation. An O-O PCF consisting of a linear combination of 70% from vdW-DF2 and 30% from low-density liquid water, as extrapolated from experiments, reproduces near-quantitatively the experimental O-O PCF for ambient water. This suggests the possibility that the new functionals may be reliable and that instead larger-scale simulations in the NPT ensemble, where the density is allowed to fluctuate in accordance with proposals for supercooled water, could resolve the apparent discrepancy with the measured PCF.
The Journal of Physical Chemistry B 08/2011; 115(48):14149-60. · 3.70 Impact Factor
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[show abstract]
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ABSTRACT: The structure of liquid water at ambient conditions is studied in ab initio
molecular dynamics simulations using van der Waals (vdW) density-functional
theory, i.e. using the new exchange-correlation functionals optPBE-vdW and
vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly
directional hydrogen-bonds, which are enhanced by standard functionals. This
brings about a softening of the microscopic structure of water, as seen from
the broadening of angular distribution functions and, in particular, from the
much lower and broader first peak in the oxygen-oxygen pair-correlation
function (PCF), indicating loss of structure in the outer solvation shells. In
combination with softer non-local correlation terms, as in the new
parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the
balance of resulting structures from open tetrahedral to more close-packed. The
resulting O-O PCF shows some resemblance with experiment for high-density water
(A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not
directly with experiment for ambient water. However, an O-O PCF consisting of a
linear combination of 70% from vdW-DF2 and 30% from experiment on low-density
liquid water reproduces near-quantitatively the experimental O-O PCF for
ambient water, indicating consistency with a two-liquid model with fluctuations
between high- and low-density regions.
01/2011;
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ABSTRACT: We study the effect of temporal correlation in a Langevin equation describing nonadiabatic dynamics at metal surfaces. For a harmonic oscillator, the Langevin equation preserves the quantum dynamics exactly and it is demonstrated that memory effects are needed in order to conserve the ground state energy of the oscillator. We then compare the result of Langevin dynamics in a harmonic potential with a perturbative master equation approach and show that the Langevin equation gives a better description in the nonperturbative range of high temperatures and large friction. Unlike the master equation, this approach is readily extended to anharmonic potentials. Using density functional theory, we calculate representative Langevin trajectories for associative desorption of N(2) from Ru(0001) and find that memory effects lower the dissipation of energy. Finally, we propose an ab initio scheme to calculate the temporal correlation function and dynamical friction within density functional theory.
The Journal of chemical physics 10/2010; 133(13):134109. · 3.09 Impact Factor
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ABSTRACT: We study the effect of temporal correlation in a Langevin equation describing non-adiabatic dynamics at metal surfaces. For a harmonic oscillator the Langevin equation preserves the quantum dynamics exactly and it is demonstrated that memory effects are needed in order to conserve the ground state energy of the oscillator. We then compare the result of Langevin dynamics in a harmonic potential with a perturbative master equation approach and show that the Langevin equation gives a better description in the non-perturbative range of high temperatures and large friction. Unlike the master equation, this approach is readily extended to anharmonic potentials. Using density functional theory we calculate representative Langevin trajectories for associative desorption of N$_2$ from Ru(0001) and find that memory effects lowers the dissipation of energy. Finally, we propose an ab-initio scheme to calculate the temporal correlation function and dynamical friction within density functional theory.
06/2010;
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ABSTRACT: A wide range of experiments have established that certain chemical reactions at metal surfaces can be driven by multiple hot-electron-mediated excitations of adsorbates. A high transient density of hot electrons is obtained by means of femtosecond laser pulses and a characteristic feature of such experiments is the emergence of a power law dependence of the reaction yield on the laser fluence Y approximately F(n). We propose a model of multiple inelastic scattering by hot electrons which reproduces this power law and the observed exponents of several experiments. All parameters are calculated within density functional theory and the delta self-consistent field method. With a simplifying assumption, the power law becomes exact and we obtain a simple physical interpretation of the exponent n, which represents the number of adsorbate vibrational states participating in the reaction.
Physical Review Letters 12/2009; 103(23):238301. · 7.37 Impact Factor
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ABSTRACT: We present a model for Desorption Induce by (Multiple) Electronic Transitions (DIET/DIMET) based on potential energy surfaces calculated with the Delta Self-Consistent Field extension of Density Functional Theory. We calculate potential energy surfaces of CO and NO molecules adsorbed on various transition metal surfaces, and show that classical nuclear dynamics does not suffice for propagation in the excited state. We present a simple Hamiltonian describing the system, with parameters obtained from the excited state potential energy surface, and show that this model can describe desorption dynamics in both the DIET and DIMET regime, and reproduce the power law behavior observed experimentally. We observe that the internal stretch degree of freedom in the molecules is crucial for the energy transfer between the hot electrons and the molecule when the coupling to the surface is strong. Comment: Typos corrected. Comment on thermal ensemble Green function added in appendix B
10/2008;
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ABSTRACT: We present a modification of the $\Delta$SCF method of calculating energies of excited states, in order to make it applicable to resonance calculations of molecules adsorbed on metal surfaces, where the molecular orbitals are highly hybridized. The $\Delta$SCF approximation is a density functional method closely resembling standard density functional theory (DFT), the only difference being that in $\Delta$SCF one or more electrons are placed in higher lying Kohn-Sham orbitals, instead of placing all electrons in the lowest possible orbitals as one does when calculating the ground state energy within standard DFT. We extend the $\Delta$SCF method by allowing excited electrons to occupy orbitals which are linear combinations of Kohn-Sham orbitals. With this extra freedom it is possible to place charge locally on adsorbed molecules in the calculations, such that resonance energies can be estimated. The method is applied to N$_2$, CO and NO adsorbed on different metallic surfaces and compared to ordinary $\Delta$SCF without our modification, spatially constrained DFT and inverse-photoemission spectroscopy (IPES) measurements. This comparison shows that the modified $\Delta$SCF method gives results in close agreement with experiment, significantly closer than the comparable methods. For N$_2$ adsorbed on ruthenium (0001) we map out a 2-dimensional part of the potential energy surfaces in the ground state and the 2$\pi$-resonance. Finally we compare the $\Delta$SCF approach on gas-phase N$_2$ and CO, to higher accuracy methods. Excitation energies are approximated with accuracy close to that of time-dependent density functional theory, and we see very good agreement in the minimum shift of the potential energy surfaces in the excited state compared to the ground state.
08/2008;
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ABSTRACT: We study the statistics of plastic rearrangement events in a simulated amorphous solid at T=0. Events are characterized by the energy release and the "slip volume", the product of plastic strain and system volume. Their distributions for a given system size L appear to be exponential, but a characteristic event size cannot be inferred, because the mean values of these quantities increase as Lalpha with alpha approximately 3/2. In contrast with results obtained in 2D models, we do not see simply connected avalanches. The exponent suggests a fractal shape of the avalanches, which is also evidenced by the mean fractal dimension and participation ratio.
Physical Review Letters 04/2007; 98(9):095501. · 7.37 Impact Factor
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ABSTRACT: We have carried out shear-deformation simulations on amorphous Mg-Cu systems at zero temperature and pressure, containing 2048-131072 atoms. At the largest size a smooth stress-strain curve is obtained with a well-defined flow stress. In the smallest system there are severe discontinuities in the stress-strain curve caused by localized plastic events. We show that the events can be characterized by a slip volume and a critical stress and we determine the distribution of these quantities from the ensemble of all events occurring in the small system. The distribution of critical stresses at which the enthalpy barriers for the individual events vanish is spread between 200 MPa and 500 MPa with a mean of 316 MPa, close to the flow stress observed in the largest system.
07/2004;
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ABSTRACT: The atomistic mechanisms of plastic deformation in amorphous metals are far from being understood. We have derived potential parameters for molecular dynamics simulations of Mg–Cu amorphous alloys using the Effective Medium Theory. We have simulated the formation of alloys by cooling from the melt, and have used these glassy configurations to carry out simulations of plastic deformation. These involved different compositions, temperatures (including zero), and types of deformation (uniaxial strain/pure shear), and yielded stress–strain curves and values of flow stress. Separate simulations were carried out to study specific features in the stress–strain curves associated with transitions involving internal rearrangements of atoms. Energy barriers were calculated as a function of stress, as was the plastic strain associated with events. The latter leads to a characteristic volume of an event which seems to correspond with the derivative of the barrier with respect to stress.
Materials Science and Engineering A. 01/2004; 387389:996-1000.
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ABSTRACT: We have obtained effective medium theory (EMT) interatomic potential parameters suitable for studying Cu-Mg metallic glasses. We present thermodynamic and structural results from simulations of such glasses over a range of compositions. We have produced low-temperature configurations by cooling from the melt at as slow a rate as practical, using constant temperature and pressure molecular dynamics. During the cooling process we have carried out thermodynamic analyses based on the temperature dependence of the enthalpy and its derivative, the specific heat, from which the glass transition temperature may be determined. We have also carried out structural analyses using the radial distribution function (RDF) and common neighbor analysis (CNA). Our analysis suggests that the splitting of the second peak, commonly associated with metallic glasses, in fact has little to do with the glass transition itself, but is simply a consequence of the narrowing of peaks associated with structural features present in the liquid state. In fact the splitting temperature for the Cu-Cu RDF is well above $T_g$. The CNA also highlights a strong similarity between the structure of the intermetallic alloys and the amorphous alloys of similar composition. We have also investigated the diffusivity in the supercooled regime. Its temperature dependence indicates fragile-liquid behavior, typical of binary metallic glasses. On the other hand, the relatively low specific heat jump of around $1.5 k_B/\mathrm{at.}$ indicates apparent strong-liquid behavior, but this can be explained by the width of the transition due to the high cooling rates.
10/2003;
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ABSTRACT: We used molecular dynamics simulations with system sizes up to 100 million atoms to simulate plastic deformation of nanocrystalline copper. By varying the grain size between 5 and 50 nanometers, we show that the flow stress and thus the strength exhibit a maximum at a grain size of 10 to 15 nanometers. This maximum is because of a shift in the microscopic deformation mechanism from dislocation-mediated plasticity in the coarse-grained material to grain boundary sliding in the nanocrystalline region. The simulations allow us to observe the mechanisms behind the grain-size dependence of the strength of polycrystalline metals.
Science 10/2003; 301(5638):1357-9. · 31.20 Impact Factor
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Science 06/2003; 300(5623):1244-5. · 31.20 Impact Factor
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ABSTRACT: In their Perspective,
Bilde-Sørensen and Schiøtz
highlight the report by
Chen
et al., whose experimental study on the deformation behavior of nanocrystalline aluminum corroborates recent simulations. The study
illustrates how the combination of computer simulations and experiment can yield important advances in understanding the atomic-scale
properties of materials. As the scale and accuracy of simulations increase, they will be increasingly used to test new materials
in "virtual laboratories."
Science 05/2003; 300(5623):1244-1245. · 31.20 Impact Factor
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Nature Material 10/2002; 1(1):15-6. · 32.84 Impact Factor
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Proceedings; 01/2002
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Physical Review B (Condensed Matter and Materials Physics). 11/1999; 60:11971.
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Nature 02/1998; 391(6667):561-563. · 36.28 Impact Factor
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Philosophical Magazine Letters 11/1996; 74(5):339-344. · 1.24 Impact Factor
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ABSTRACT: In the present study we have used in situ EXAFS to provide experimental evidence for a reversible change in the morphology of the metallic particles in a high surface area, porous catalyst system, Cu/ZnO, containing small metallic copper particles. By changing the oxidation potential in the synthesis gas mixture, it is found that the apparent Cu-Cu coordination number changes in an essentially reversible manner suggesting that the small metallic Cu particles dynamically change morphology. This indicates that a wetting/non-wetting phenomenon takes place in the Cu/ZnO system with changing partial pressures of oxygen. Under similar conditions, these effects are not observed when copper is supported on, for example, SiO2. A model based on surface and interface free energies provides a simple explanation for the observed results. Since the wetting/non-wetting processes are accompanied by a change in the active surface area, the observed behavior has important general implications and such effects must be incorporated into microkinetic models in order to provide a proper description of the catalyst performance.
Topics in Catalysis 08/1994; 1(3):367-376. · 2.62 Impact Factor
