708 research outputs found
Low-pressure phase diagram of crystalline benzene from quantum Monte Carlo
We studied the low-pressure (0–10 GPa) phase diagram of crystalline benzene using quantum Monte Carlo and density functional theory (DFT) methods. We performed diffusion quantum Monte Carlo (DMC) calculations to obtain accurate static phase diagrams as benchmarks for modern van der Waals density functionals. Using density functional perturbation theory, we computed the phonon contributions to the free energies. Our DFT enthalpy-pressure phase diagrams indicate that the Pbca and P21/c structures are the most stable phases within the studied pressure range. The DMC Gibbs free-energy calculations predict that the room temperature Pbca to P21/c phase transition occurs at 2.1(1) GPa. This prediction is consistent with available experimental results at room temperature. Our DMC calculations give 50.6 ± 0.5 kJ/mol for crystalline benzene lattice energy
Systematically convergent method for accurate total energy calculations with localized atomic orbitals
We introduce a method for solving a self consistent electronic calculation
within localized atomic orbitals, that allows us to converge to the complete
basis set (CBS) limit in a stable, controlled, and systematic way. We compare
our results with the ones obtained with a standard quantum chemistry package
for the simple benzene molecule. We find perfect agreement for small basis set
and show that, within our scheme, it is possible to work with a very large
basis in an efficient and stable way. Therefore we can avoid to introduce any
extrapolation to reach the CBS limit. In our study we have also carried out
variational Monte Carlo (VMC) and lattice regularized diffusion Monte Carlo
(LRDMC) with a standard many-body wave function (WF) defined by the product of
a Slater determinant and a Jastrow factor. Once the Jastrow factor is optimized
by keeping fixed the Slater determinant provided by our new scheme, we obtain a
very good description of the atomization energy of the benzene molecule only
when the basis of atomic orbitals is large enough and close to the CBS limit,
yielding the lowest variational energies.Comment: 22 pages, 6 figures, accepted in Physical Review
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
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
Efficient method for grand-canonical twist averaging in quantum Monte Carlo calculations
We introduce a simple but efficient method for grand-canonical twist averaging in quantum Monte Carlo calculations. By evaluating the thermodynamic grand potential instead of the ground state total energy, we greatly reduce the sampling errors caused by twist-dependent fluctuations in the particle number. We apply this method to the electron gas and to metallic lithium, aluminum, and solid atomic hydrogen. We show that, even when using a small number of twists, grand-canonical twist averaging of the grand potential produces better estimates of ground state energies than the widely used canonical twist-averaging approach
Lateral Antebrachial Cutaneous Nerve injury induced by phlebotomy
BACKGROUND: Phlebotomy is one of the routine procedures done in medical labs daily. CASE PRESENTATION: A 52 yr woman noted shooting pain and dysesthesia over her right side anterolateral aspect of forearm, clinical examination and electrodiagnostic studies showed severe involvement of right side lateral antebrachial cutaneous nerve. CONCLUSION: Phlebotomy around lateral aspect of antecubital fossa may cause lateral antebrachial cutaneous nerve injury, electrodiagnostic studies are needed for definite diagnosis
The role of van der Waals and exchange interactions in high-pressure solid hydrogen
We investigate the van der Waals interactions in solid molecular hydrogen structures. We calculate enthalpy and the Gibbs free energy to obtain zero and finite temperature phase diagrams, respectively. We employ density functional theory (DFT) to calculate the electronic structure and density functional perturbation theory (DFPT) with van der Waals (vdW) functionals to obtain phonon spectra. We focus on the solid molecular C2/c, Cmca-12, P63/m, Cmca, and Pbcn structures within the pressure range of 200 < P < 450 GPa. We propose two structures of the C2/c and Pbcn for phase III which are stabilized within different pressure range above 200 GPa. We find that vdW functionals have a big effect on vibrations and finite-temperature phase stability, however, different vdW functionals have different effects. We conclude that, in addition to the vdW interaction, a correct treatment of the high charge gradient limit is essential. We show that the dependence of molecular bond-lengths on exchange–correlation also has a considerable influence on the calculated metallization pressure, introducing errors of up to 100 GPa
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
Seed Germination, Seedling Growth and Enzyme Activity of Wheat Seed Primed under Drought and Different Temperature Conditions
The study aimed was to determine the effects of drought stress (0, -4, -8, -12 bar) and osmopriming (-15 bar PEG 6000 for 15 at 24 h) on seed germination, seedling growth and enzyme activity at different temperatures were assessed in the laboratory for wheat. Results showed that the highest germination percentage (GP) (94.33%), normal seedling percentage (NSP) (92%), germination index (GI) (44.85) and seedling length (11.03 cm) were attained from osmo-priming in control conditions. Therefore, seed priming with PEG 6000 significantly (p≤ 0.01) increased germination characteristics as compared to the unprimed seeds under drought stress. Also, osmopriming increased catalase (CAT) and ascorbate peroxidase (APX) as compared to the unprimed
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