2,668 research outputs found
Quantum Monte Carlo calculations of H dissociation on Si(001)
We present quantum Monte Carlo calculations for various reaction pathways of
H with Si(001), using large model clusters of the surface. We obtain
reaction energies and energy barriers noticeably higher than those from
approximate exchange-correlation functionals. In improvement over previous
studies, our adsorption barriers closely agree with experimental data. For
desorption, the calculations give barriers for conventional pathways in excess
of the presently accepted experimental value, and pinpoint the role of coverage
effects and desorption from steps.Comment: 4 pages, 1 figur
Ferromagnetism and magneto-dielectric effect in insulating LaBiMn4/3Co2/3O6 thin films
High quality epitaxial thin films of LaBiMn4/3Co2/3O6 perovskite were
fabricated on (001)-oriented SrTiO3 and LaAlO3 substrates by the pulsed laser
deposition technique. Magnetization measurements reveal a strong magnetic
anisotropy and a ferromagnetic behavior that is in agreement with a
super-exchange interaction between Mn4+ and Co2+ ions, which are randomly
distributed in the B-site. A distinct anomaly is observed in the dielectric
measurements at 130K corresponding to the onset of the magnetic ordering,
suggesting a coupling. Above this temperature, the extrinsic Maxwell-Wagner
effect is dominating. Theses results are explained using the Raman
spectroscopic studies indicating a weak spin-lattice interaction around this
magnetic transition.Comment: Submitted to Appl. Phys. Lett. (2008
Correlated sampling in quantum Monte Carlo: a route to forces
In order to find the equilibrium geometries of molecules and solids and to
perform ab initio molecular dynamics, it is necessary to calculate the forces
on the nuclei. We present a correlated sampling method to efficiently calculate
numerical forces and potential energy surfaces in diffusion Monte Carlo. It
employs a novel coordinate transformation, earlier used in variational Monte
Carlo, to greatly reduce the statistical error. Results are presented for
first-row diatomic molecules.Comment: 5 pages, 2 postscript figure
The role of electronic correlation in the Si(100) reconstruction: a quantum Monte Carlo study
Recent low-temperature scanning tunneling experiments have challenged the
generally accepted picture of buckled silicon dimers as the ground state
reconstruction of the Si(100) surface. Together with the symmetric dimer model
of the surface suggested by quantum chemistry calculations on small clusters,
these findings question our general understanding of electronic correlations at
surfaces and its proper description within density functional theory. We
present quantum Monte Carlo calculations on large cluster models of the
symmetric and buckled surface, and conclude that buckling remains energetically
more favorable even when the present-day best treatment of electronic
correlation is employed.Comment: 5 pages, Revtex, 10 figure
Potholes and molehills: bias in the diagnostic performance of diffusion-tensor imaging in concussion
PURPOSE: To investigate the extent of bias in a clinical study involving pothole analysis of diffusion-tensor imaging (DTI) data used to quantify white matter lesion load in diseases with a heterogeneous spatial distribution of pathologic findings, such as mild traumatic brain injury (TBI), and create a mathematical model of the bias. MATERIALS AND METHODS: Use of the same reference population to define normal findings and make comparisons with a patient group introduces bias, which potentially inflates reported diagnostic performance. In this institutional review board-approved prospective observational cohort study, DTI data were obtained in 20 patients admitted to the emergency department with mild TBI and in 16 control subjects. Potholes and molehills were defined as clusters of voxels with fractional anisotropy values more than 2 standard deviations below and above the mean of the corresponding voxels in the reference population, respectively. The number and volume of potholes and molehills in the two groups were compared by using a Mann-Whitney U test. RESULTS: Standard analysis showed significantly more potholes in mild TBI than in the control group (102.5 +/- 34.3 vs 50.6 +/- 28.9, P \u3c .001). Repeat analysis by using leave-one-out cross-validation decreased the apparent difference in potholes between groups (mild TBI group, 102.5 +/- 34.3; control group, 93.4 +/- 27.2; P = .369). It was demonstrated that even with 100 subjects, this bias can decrease the voxelwise false-positive rate by more than 30% in the control group. CONCLUSION: The pothole approach to neuroimaging data may introduce bias, which can be minimized by independent training and test groups or cross-validation methods. This bias is sufficient to call into question the previously reported diagnostic performance of DTI for mild TBI
Monte Carlo energy and variance minimization techniques for optimizing many-body wave functions
We investigate Monte Carlo energy and variance minimization techniques for
optimizing many-body wave functions. Several variants of the basic techniques
are studied, including limiting the variations in the weighting factors which
arise in correlated sampling estimations of the energy and its variance. We
investigate the numerical stability of the techniques and identify two reasons
why variance minimization exhibits superior numerical stability to energy
minimization. The characteristics of each method are studied using a
non-interacting 64-electron model of crystalline silicon. While our main
interest is in solid state systems, the issues investigated are relevant to
Monte Carlo studies of atoms, molecules and solids. We identify a robust and
efficient variance minimization scheme for optimizing wave functions for large
systems.Comment: 14 pages, including 7 figures. To appear in Phys. Rev. B. For related
publications see http://www.tcm.phy.cam.ac.uk/Publications/many_body.htm
Comparing the Weighted Density Approximation with the LDA and GGA for Ground State Properties of Ferroelectric Perovskites
First-principles calculations within the weighted density approximation (WDA)
were performed for ground state properties of ferroelectric perovskites
PbTiO, BaTiO, SrTiO, KNbO and KTaO. We used the plane-wave
pseudopotential method, a pair distribution function based on the uniform
electron gas, and shell partitioning. Comparing with the local density
approximation (LDA) and the general gradient approximation (GGA), we found that
the WDA significantly improves the equilibrium volume of these materials in
cubic symmetry over both the LDA and GGA; Ferroelectric instabilities
calculated by the WDA agree with the LDA and GGA very well; At the experimental
ferroelectric lattice, optimized atom positions by the WDA are in good
agreement with measured data; However the WDA overestimates the strain of
tetragonal PbTiO at experimental volume; The WDA overestimates the volume
of fully relaxed structures, but the GGA results are even worse. Some
calculations were also done with other models for . It is found that a
with longer range behavior yields improved relaxed structures. Possible avenues
for improving the WDA are discussed.Comment: 19 pages, 3 figures, submitted to PR
Transition metal oxides using quantum Monte Carlo
The transition metal-oxygen bond appears prominently throughout chemistry and
solid-state physics. Many materials, from biomolecules to ferroelectrics to the
components of supernova remnants contain this bond in some form. Many of these
materials' properties strongly depend on fine details of the TM-O bond and
intricate correlation effects, which make accurate calculations of their
properties very challenging. We present quantum Monte Carlo, an explicitly
correlated class of methods, to improve the accuracy of electronic structure
calculations over more traditional methods like density functional theory. We
find that unlike s-p type bonding, the amount of hybridization of the d-p bond
in TM-O materials is strongly dependant on electronic correlation.Comment: 20 pages, 4 figures, to appear as a topical review in J. Physics:
Condensed Matte
Manual de biossegurança em instalações com certificado de qualidade em biossegurança na Embrapa Arroz e Feijão.
Foi produzido o presente manual de biossegurança para a orientação nas diversas atividades desenvolvidas nas dependências regidas pelo Certificado de Qualidade em Biossegurança (CQB). Esclarece o significado de termos técnicos úteis empregados e apresenta um conjunto de normas e procedimentos que certamente minimizarão os riscos de trabalho e de uma possÃvel liberação acidental de organismos geneticamente modificados.bitstream/CNPAF/26693/1/doc_216.pd
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