944 research outputs found
Fate of density functional theory in high-pressure solid hydrogen
This paper investigates some of the successes and failures of density
functional theory in the study of high-pressure solid hydrogen at low
temperature. We calculate the phase diagram, metallization pressure, phonon
spectrum, and proton zero-point energy using three popular exchange-correlation
functionals: the local density approximation (LDA), the Perdew-Burke-Ernzerhof
(PBE) generalized gradient approximation, and the semi-local
Becke-Lee-Yang-Parr (BLYP) functional. We focus on the solid molecular
P/m, C2/c, Cmca-12, and Cmca structures in the pressure range from
GPa over which phases I, II and III are observed experimentally. At
the static level of theory, in which proton zero-point energy is ignored, the
LDA, PBE and BLYP functionals give very different structural transition and
metallization pressures, with the BLYP phase diagram in better agreement with
experiment. Nevertheless, all three functionals provide qualitatively the same
information about the band gaps of the four structures and the phase
transitions between them. Going beyond the static level, we find that the
frequencies of the vibron modes observed above 3000 cm depend strongly
on the choice of exchange-correlation functional, although the low-frequency
part of the phonon spectrum is little affected. The largest and smallest values
of the proton zero-point energy, obtained using the BLYP and LDA functionals,
respectively, differ by more than 10 meV/proton. Including the proton
zero-point energy calculated from the phonon spectrum within the harmonic
approximation improves the agreement of the BLYP and PBE phase diagrams with
experiment. Taken as a whole, our results demonstrate the inadequacy of
mean-field-like density functional calculations of solid molecular hydrogen in
phases I, II and III and emphasize the need for more sophisticated methods.Comment: Accepted for publicatio
Systematic study of finite-size effects in quantum Monte Carlo calculations of real metallic systems
Quantum Monte Carlo Study of High Pressure Solid Molecular Hydrogen
We use the diffusion quantum Monte Carlo (DMC) method to calculate the ground
state phase diagram of solid molecular hydrogen and examine the stability of
the most important insulating phases relative to metallic crystalline molecular
hydrogen. We develop a new method to account for finite-size errors by
combining the use of twist-averaged boundary conditions with corrections
obtained using the Kwee-Zhang-Krakauer (KZK) functional in density functional
theory. To study band-gap closure and find the metallization pressure, we
perform accurate quasi-particle many-body calculations using the method.
In the static approximation, our DMC simulations indicate a transition from the
insulating Cmca-12 structure to the metallic Cmca structure at around 375 GPa.
The band gap of Cmca-12 closes at roughly the same pressure. In the
dynamic DMC phase diagram, which includes the effects of zero-point energy, the
Cmca-12 structure remains stable up to 430 GPa, well above the pressure at
which the band gap closes. Our results predict that the semimetallic state
observed experimentally at around 360 GPa [Phys. Rev. Lett. {\bf 108}, 146402
(2012)] may correspond to the Cmca-12 structure near the pressure at which the
band gap closes. The dynamic DMC phase diagram indicates that the hexagonal
close packed structure, which has the largest band gap of the
insulating structures considered, is stable up to 220 GPa. This is consistent
with recent X-ray data taken at pressures up to 183 GPa [Phys. Rev. B {\bf 82},
060101(R) (2010)], which also reported a hexagonal close packed arrangement of
hydrogen molecules
Dissociation of high-pressure solid molecular hydrogen: Quantum Monte Carlo and anharmonic vibrational study
A theoretical study is reported of the molecular-to-atomic transition in
solid hydrogen at high pressure. We use the diffusion quantum Monte Carlo
method to calculate the static lattice energies of the competing phases and a
density-functional-theory-based vibrational self-consistent field method to
calculate anharmonic vibrational properties. We find a small but significant
contribution to the vibrational energy from anharmonicity. A transition from
the molecular Cmca-12 direct to the atomic I4_1/amd phase is found at 374 GPa.
The vibrational contribution lowers the transition pressure by 91 GPa. The
dissociation pressure is not very sensitive to the isotopic composition. Our
results suggest that quantum melting occurs at finite temperature.Comment: Accepted for publication by Phys. Rev. Let
Direct Dry Carbonation of Mining and Industrial Wastes in a Fluidized Bed for Offsetting Carbon Emissions
The direct dry mineral carbonation of selected mining and industrial wastes, using carbon dioxide derived from combustion flue gas, was evaluated. Specifically, coal fly ash from two Australian brown coal-fired power plants, red mud from the refinement of bauxite into alumina, and diamond tailings were considered, due to their relevant residual alkali content. These materials were tested in a laboratory-scale fluidized bed reactor at different temperatures (300–450° C), in a reactive environment that simulated the typical CO2 concentration in a combustion flue gas. The experimental results showed a low, but still appreciable, CO2 capture capacity for three of the tested materials, which appears to be more favorable in the lower temperature range and with relatively fast kinetics, indicating the practical relevance of the process. One of the fly ashes exhibited a different behavior; starting at 350° C, the sorbent began to release CO2, rather than absorb it. This suggested that the sorbent was already extensively carbonated by weathering before the tests. This study provides some evidence for the possible viability of recycling mining waste and for the circular economy in offsetting carbon emissions in the mining industry
Using Bayesian Model Averaging (BMA) to calibrate probabilistic surface temperature forecasts over Iran
Using Bayesian Model Averaging (BMA), an attempt was made to obtain
calibrated probabilistic numerical forecasts of 2-m temperature over
Iran. The ensemble employs three limited area models (WRF, MM5 and HRM),
with WRF used with five different configurations. Initial and boundary
conditions for MM5 and WRF are obtained from the National Centers for
Environmental Prediction (NCEP) Global Forecast System (GFS) and for HRM the
initial and boundary conditions come from analysis of Global Model Europe
(GME) of the German Weather Service. The resulting ensemble of seven members
was run for a period of 6 months (from December 2008 to May 2009) over Iran.
The 48-h raw ensemble outputs were calibrated using BMA technique for 120
days using a 40 days training sample of forecasts and relative verification
data.
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The calibrated probabilistic forecasts were assessed using rank histogram
and attribute diagrams. Results showed that application of BMA improved the
reliability of the raw ensemble. Using the weighted ensemble mean forecast
as a deterministic forecast it was found that the deterministic-style BMA
forecasts performed usually better than the best member's deterministic
forecast
Resonating Valence Bond wave function with molecular orbitals: application to first-row molecules
We introduce a method for accurate quantum chemical calculations based on a simple variational wave function, defined by a single geminal that couples all the electrons into singlet pairs, combined with a real space correlation factor. The method uses a constrained variational optimization, based on an expansion of the geminal in terms of molecular orbitals. It is shown that the most relevant non-dynamical correlations are correctly reproduced once an appropriate number n of molecular orbitals is considered. The value of n is determined by requiring that, in the atomization limit, the atoms are described by Hartree-Fock Slater determinants with Jastrow correlations. The energetics, as well as other physical and chemical properties, are then given by an efficient variational approach based on standard quantum Monte Carlo techniques. We test this method on a set of homonuclear (Be2, B2, C2, N2, O2, and F2) and heteronuclear (LiF, and CN) dimers for which strong non-dynamical correlations and/or weak van der Waals interactions are present
Perfect essential graphs
Let R be a commutative ring with identity, and let Z(R) be the set of zero-divisors of R. Let EG(R) be a simple undirect graph associated with R whose vertex set is the set of all nonzero zero-divisors of R and and two distinct vertices x, y in this graph are joined by an edge if and only if AnnR(xy) is an essential ideal. A perfect graph is a graph in which the chromatic number of every induced subgraph equals the size of the largest clique of that subgraph. In this paper, we characterize all rings whose EG(R) is perfect
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