1,721,296 research outputs found
Hydrogen dissociation on Mg(0001) studied via quantum Monte Carlo calculations
No full textWe have used diffusion Monte Carlo (DMC) simulations to calculate the energy barrier for H(2) dissociation on the Mg(0001) surface. The calculations employ pseudopotentials and systematically improvable B-spline basis sets to expand the single-particle orbitals used to construct the trial wave functions. Extensive tests on system size, time step, and other sources of errors, performed on periodically repeated systems of up to 550 atoms, show that all these errors together can be reduced to similar to 0.03 eV. The DMC dissociation barrier is calculated to be 1.18 +/- 0.03 eV and is compared to those obtained with density-functional theory using various exchange-correlation functionals, with values ranging between 0.44 and 1.07 eV
Melting curve of face-centered-cubic nickel from first-principles calculations
Full text linkThe melting curve of Ni up to 100 GPa has been calculated using first-principles methods based on density functional theory (DFT). We used two complementary approaches: (i) coexistence simulations with a reference system and then free-energy corrections between DFT and the reference system, and (ii) direct DFT coexistence using simulation cells including 1000 atoms. The calculated zero pressure melting temperature is slightly underestimated at 1637±10 K (experimental value is 1728 K), and at high pressure is significantly higher than recent measurements in diamond-anvil cell experiments [ Phys. Rev. B 87 054108 (2013)]. The zero pressure DFT melting slope is calculated to be 30±2 K, in good agreement with the experimental value of 28 K
Structural properties and enthalpy of formation of magnesium hydride from quantum Monte Carlo calculations
No full textWe have used diffusion Monte Carlo (DMC) calculations to study the structural properties of magnesium hydride (MgH(2)), including the pressure-volume equation of state, the cohesive energy, and the enthalpy of formation from magnesium bulk and hydrogen gas. The calculations employ pseudopotentials and B-spline basis sets to expand the single particle orbitals used to construct the trial wave functions. Extensive tests on system size, time step, and other sources of errors, performed on periodically repeated systems of up to 1050 atoms, show that all these errors together can be reduced to below 10 meV/f.u.. We find excellent agreement with the experiments for the equilibrium volume of both the Mg and the MgH(2) crystals. The cohesive energy of the Mg crystal is found to be 1.51 (1) eV and agrees perfectly with the experimental value of 1.51 eV. The enthalpy of formation of MgH(2) from Mg bulk and H(2) gas is found to be 0.85 +/- 0.01 eV/f.u., or 82 +/- 1 kJ/mole, which is off the experimental one of 76.1 +/- 1 kJ/mole only by 6 kJ/mole. This shows that DMC can almost achieve chemical accuracy (1 kcal/mole) on this system. Density functional theory errors are shown to be much larger and depend strongly on the functional employed
Hydrogen dissociation and diffusion on transition metal (= Ti, Zr, V, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces
No full textThe kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H(2) molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab initio density functional theory investigation of H(2) dissociation and subsequent atomic H diffusion on TM (= Ti, V, Zr, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TMs on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TMs on the right of the periodic table do not bind H, however, they do not reduce the barrier to dissociate H(2) significantly. our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved
The role of steps in the dissociation of H-2 on Mg(0001)
No full textThe role of steps in the dissociation of molecules on metal surfaces has been extensively investigated in the past. In particular, both theoretical calculations and experimental results for H-2 dissociation on transition metal (TM) surfaces show that steps can significantly increase the reactivity, leading to higher metal-H binding energies and lower activation energies. Here we have used density functional theory (DFT) with the generalized gradient approximation (GGA) to investigate the role of steps on the Mg(0001) surface in the dissociation of H-2 and the binding of H to the metal surface. Our results follow those found for TM surfaces as far as H adsorption energies are concerned, namely that adsorption energies are higher near the steps. However, we find that the activation energy for the dissociation of hydrogen is hardly affected by the presence of steps, with a DFT-GGA value of 0.85 eV, only marginally lower than the value 0.87 eV found on the flat Mg(0001) surface
The Studium Generale Program and the Effectiveness of the History of Concepts
No full textThe process initiated by the Bologna Conference in 1999 towards an economically, environmentally, and culturally sustainable development has led to the constitution of the European Research Area, in which education is in the process of being streamlined in all participating countries. One of its first consequences was a long wished for increase in the number of graduates all over Europe. Given that the main difficulty seems to lie in disseminating an idea of science that were at the crossroad between basic and applied research, very much in the spirit of the Bologna declaration, an answer to this difficulty is the Studium Generale Program (=SGP), which is a set of interdisciplinary modules aimed at presenting to students of all disciplines the nucleus of European science and philosophy (from Aristotle’s Analytica to Euclid’s Stoicheia, from Plato’s Politeia to Augustine’s Confessiones). Developed since 2005 by a network of European universities brought together by the Guardini Stiftung e.V. (Berlin) with funding of Germany’s Bundesministerium für Bildung und Forschung (Bonn), the SGP is close to completion. It is the best instrument for achieving this dissemination goal in as far as it enables students and teachers to find their own ways within Europe’s intellectual identity. First and foremost, then, it is the methods and the contents produced by the SGP that provides an answer to the question, “What does it mean to be European?” This is the key role the SGP plays with respect of the constitution of Europe’s polycentric identity
Towards reconciling experimental and computational determinations of Earth's core thermal conductivity
No full textThe thermal conductivity (κ) of Earth's core is a critical parameter that controls predictions of core cooling rate, inner core age and the power available to the geodynamo. However, the values of core thermal conductivity inferred from recent studies span a wide range due to the challenges of extrapolating to the pressure-temperature-composition (P-T-C) conditions of the core liquid. In particular, extrapolations of κ from direct experimental determinations are lower than ab initio calculations conducted at core conditions. We have performed density functional theory (DFT) calculations to determine the thermal conductivity and resistivity (ρ) of solid FeSi alloys with two compositions, 4 mol % and 15 mol % Si, at a range of temperatures (850-4350 K) and pressures (60-144 GPa) for ease of comparison with recent directly measured κ values. In agreement with recent experiments, our calculations show that for the larger Si composition the resistivity of the mixture increases substantially, compared to pure Fe, reaching its saturated value already at the lowest temperature investigated. As a result, the thermal conductivity of the mixture is also correspondingly reduced. We also analysed the effect of possible errors in the DFT calculations due to the neglect of electron-electron scattering (EES) processes. Our results show that experimental and EES-corrected DFT calculations of κ are actually consistent within uncertainties when compared directly at overlapping P-T-C conditions. We present new core thermal history models using our EES-corrected estimates of κ=75−81 W m−1 K−1 at core-mantle boundary (CMB) conditions, which support previous determinations of late inner core formation around 400-700 Myrs ago and an early molten lower mantle
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