Ab initio melting curve of molybdenum by the phase coexistence method

We report ab initio calculations of the melting curve of molybdenum for the pressure range 0-400 GPa. The calculations employ density functional theory (DFT) with the Perdew-Burke-Ernzerhof exchange-correlation functional in the projector augmented wave (PAW) implementation. We present tests showing that these techniques accurately reproduce experimental data on low-temperature b.c.c. Mo, and that PAW agrees closely with results from the full-potential linearized augmented plane-wave implementation. The work attempts to overcome the uncertainties inherent in earlier DFT calculations of the melting curve of Mo, by using the ``reference coexistence'' technique to determine the melting curve. In this technique, an empirical reference model (here, the embedded-atom model) is accurately fitted to DFT molecular dynamics data on the liquid and the high-temperature solid, the melting curve of the reference model is determined by simulations of coexisting solid and liquid, and the ab initio melting curve is obtained by applying free-energy corrections. Our calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high pressure melting curve deduced from static compression experiments. Calculated results for the radial distribution function show that the short-range atomic order of the liquid is very similar to that of the high-T solid, with a slight decrease of coordination number on passing from solid to liquid. The electronic densities of states in the two phases show only small differences. The results do not support a recent theory according to which very low dTm/dP values are expected for b.c.c. transition metals because of electron redistribution between s-p and d states.Comment: 27 pages, 10 figures. to be published in Journal of Chemical Physic

Quantum atomic delocalization vs. structural disorder in amorphous silicon

Quantum effects on the atom delocalization in amorphous silicon have been studied by path-integral Monte Carlo simulations from 30 to 800 K. The quantum delocalization is appreciable vs. topological disorder, as seen from structural observables such as the radial distribution function (RDF). At low temperatures, the width of the first peak in the RDF increases by a factor of 1.5 due to quantum effects. The overall anharmonicity of the solid vibrations at finite temperatures in amorphous silicon is clearly larger than in the crystalline material. Low-energy vibrational modes are mainly located on coordination defects in the amorphous material.Comment: 5 pages, 5 PS figures, REVTE

Bulk and surface energetics of lithium hydride crystal: benchmarks from quantum Monte Carlo and quantum chemistry

We show how accurate benchmark values of the surface formation energy of crystalline lithium hydride can be computed by the complementary techniques of quantum Monte Carlo (QMC) and wavefunction-based molecular quantum chemistry. To demonstrate the high accuracy of the QMC techniques, we present a detailed study of the energetics of the bulk LiH crystal, using both pseudopotential and all-electron approaches. We show that the equilibrium lattice parameter agrees with experiment to within 0.03 %, which is around the experimental uncertainty, and the cohesive energy agrees to within around 10 meV per formula unit. QMC in periodic slab geometry is used to compute the formation energy of the LiH (001) surface, and we show that the value can be accurately converged with respect to slab thickness and other technical parameters. The quantum chemistry calculations build on the recently developed hierarchical scheme for computing the correlation energy of a crystal to high precision. We show that the hierarchical scheme allows the accurate calculation of the surface formation energy, and we present results that are well converged with respect to basis set and with respect to the level of correlation treatment. The QMC and hierarchical results for the surface formation energy agree to within about 1 %.Comment: 16 pages, 4 figure

Adsorption of hydrogen sulphide over rhodium/silica and rhodium/alumina at 293 and 873 K, with co-adsorption of carbon monoxide and hydrogen

In this study, we have examined the adsorption of hydrogen sulphide and carbon monoxide over rhodium/silica and rhodium/alumina catalysts. Adsorption of hydrogen sulphide was measured at 293 and 873 K and at 873 K in a 1:1 ratio with hydrogen. At 293 K, over Rh/silica, hydrogen sulphide adsorption capacity was similar to that of carbon monoxide; however, over Rh/alumina, the carbon monoxide adsorption capacity was higher, probably due to the formation of RhI(CO)2. Over Rh/silica, the primary adsorbed state was HS(ads), in contrast to Rh/alumina, where the H2:S ratio was 1:1 indicating that the adsorbed state was S(ads). Competitive adsorption between CO and H2S over Rh/silica and Rh/alumina revealed adsorption sites on the metal that only adsorbed carbon monoxide, only adsorbed hydrogen sulphide or could adsorb both species. At 873 K, hydrogen sulphide adsorption produced the bulk sulphide Rh2S3; however, when a 1:1 H2:H2S mixture was used formation of the bulk sulphide was inhibited and a reduced amount of hydrogen sulphide was adsorbed

Melting curve and Hugoniot of molybdenum up to 400 GPa by ab initio simulations

We report ab initio calculations of the melting curve and Hugoniot of molybdenum for the pressure range 0-400 GPa, using density functional theory (DFT) in the projector augmented wave (PAW) implementation. We use the ``reference coexistence'' technique to overcome uncertainties inherent in earlier DFT calculations of the melting curve of Mo. Our calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high pressure melting curve from static compression experiments. Our calculated P(V) and T(P) Hugoniot relations agree well with shock measurements. We use calculations of phonon dispersion relations as a function of pressure to eliminate some possible interpretations of the solid-solid phase transition observed in shock experiments on Mo.Comment: 8 pages, 6 figure

A comparison between quantum chemistry and quantum Monte Carlo techniques for the adsorption of water on the (001) LiH surface

We present a comprehensive benchmark study of the adsorption energy of a single water molecule on the (001) LiH surface using periodic coupled cluster and quantum Monte Carlo theories. We benchmark and compare different implementations of quantum chemical wave function based theories in order to verify the reliability of the predicted adsorption energies and the employed approximations. Furthermore we compare the predicted adsorption energies to those obtained employing widely-used van der Waals density-functionals. Our findings show that quantum chemical approaches are becoming a robust and reliable tool for condensed phase electronic structure calculations, providing an additional tool that can also help in potentially improving currently available van der Waals density-functionals

A Self-Consistent First-Principles Technique Having Linear Scaling

An algorithm for first-principles electronic structure calculations having a computational cost which scales linearly with the system size is presented. Our method exploits the real-space localization of the density matrix, and in this respect it is related to the technique of Li, Nunes and Vanderbilt. The density matrix is expressed in terms of localized support functions, and a matrix of variational parameters, L, having a finite spatial range. The total energy is minimized with respect to both the support functions and the elements of the L matrix. The method is variational, and becomes exact as the ranges of the support functions and the L matrix are increased. We have tested the method on crystalline silicon systems containing up to 216 atoms, and we discuss some of these results.Comment: 12 pages, REVTeX, 2 figure

Probing Disordered Substrates by Imaging the Adsorbate in its Fluid Phase

Several recent imaging experiments access the equilibrium density profiles of interacting particles confined to a two-dimensional substrate. When these particles are in a fluid phase, we show that such data yields precise information regarding substrate disorder as reflected in one-point functions and two-point correlations of the fluid. Using Monte Carlo simulations and replica generalizations of liquid state theories, we extract unusual two-point correlations of time-averaged density inhomogeneities induced by disorder. Distribution functions such as these have not hitherto been measured but should be experimentally accessible.Comment: 10 pages revtex 4 figure