Quantum Monte Carlo calculations of H2_2 dissociation on Si(001)

We present quantum Monte Carlo calculations for various reaction pathways of H$_2$ 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$_3$, BaTiO$_3$, SrTiO$_3$, KNbO$_3$ and KTaO$_3$. We used the plane-wave pseudopotential method, a pair distribution function $G$ 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$_3$ 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 $G$. It is found that a $G$ 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 possível liberação acidental de organismos geneticamente modificados.bitstream/CNPAF/26693/1/doc_216.pd