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Monte Carlo Calculations (monte + carlo_calculation)
Selected AbstractsThermodynamics of Hydrogen and Hydrogen-Helium Plasmas: Path Integral Monte Carlo Calculations and Chemical PictureCONTRIBUTIONS TO PLASMA PHYSICS, Issue 3-4 2005V. S. Filinov Abstract In this paper we study thermodynamic properties of hydrogen and hydrogen-helium mixtures with the help of the direct path integral Monte Carlo simulations. The results are compared with available theoretical and experimental methods based, in particular, on chemical picture. We investigate the effects of temperature ionization in low-density hydrogen plasma. We also present a number of calculated isotherms for hydrogenhelium mixture with the mass concentration of helium Y = 0.234 in the range from 104 K to 2 · 105 K. In the density region where a sharp conductivity rise have been observed experimentally the simulations give indications for one or two plasma phase transitions, in accordance with earlier theoretical predictions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Exact Fixed-node Quantum Monte Carlo: Differential ApproachCHINESE JOURNAL OF CHEMISTRY, Issue 11 2005Hong-Xin Huang Abstract A differential approach for exact fixed-node quantum Monte Carlo calculation was proposed in this paper. This new algorithm can be used to directly compute the energy differential between two systems in exact fixed-node quantum Monte Carlo process, making the statistical error of calculation reduce to order of 10,2 kJ/mol and recover about more than 90% of the correlation energy. The approach was employed to set up a potential energy surface of a molecule, through a model of rigid move, and Jacobi transformation utilized to make energy calculation for two configurations of a molecule having good positive correlation. So, an accurate energy differential could be obtained, and the potential energy surface with good quality depicted. This novel algorithm was used to study the potential energy curve of the ground state of BH and the potential energy surface of H3, and could be also applied to study other related fields such as molecular spectroscopy and the energy variation of chemical reactions. [source] Improvements in the 2D TEP Neutral Particle Transport Calculation in Edge PlasmasCONTRIBUTIONS TO PLASMA PHYSICS, Issue 7-9 2006D. Zhang Abstract Extensions of the 2D Transmission and Escape Probability neutral particle transport method in treating the spatial non-uniformity of collision sources and neutral energy effects are presented. These extensions have been tested by benchmarks against Monte Carlo calculations for specially designed models and for realistic DIII-D discharges. The comparisons indicate these extensions improve accuracy of the TEP method. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effects of Measurement Error on Horizontal Hydraulic Gradient EstimatesGROUND WATER, Issue 1 2007J.F. Devlin During the design of a natural gradient tracer experiment, it was noticed that the hydraulic gradient was too small to measure reliably on an ,500-m2 site. Additional wells were installed to increase the monitored area to 26,500 m2, and wells were instrumented with pressure transducers. The resulting monitoring system was capable of measuring heads with a precision of ±1.3 × 10,2 m. This measurement error was incorporated into Monte Carlo calculations, in which only hydraulic head values were varied between realizations. The standard deviation in the estimated gradient and the flow direction angle from the x-axis (east direction) were calculated. The data yielded an average hydraulic gradient of 4.5 × 10,4±25% with a flow direction of 56° southeast ±18°, with the variations representing 1 standard deviation. Further Monte Carlo calculations investigated the effects of number of wells, aspect ratio of the monitored area, and the size of the monitored area on the previously mentioned uncertainties. The exercise showed that monitored areas must exceed a size determined by the magnitude of the measurement error if meaningful gradient estimates and flow directions are to be obtained. The aspect ratio of the monitored zone should be as close to 1 as possible, although departures as great as 0.5 to 2 did not degrade the quality of the data unduly. Numbers of wells beyond three to five provided little advantage. These conclusions were supported for the general case with a preliminary theoretical analysis. [source] Comparison of global and local sensitivity techniques for rate constants determined using complex reaction mechanismsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2001James J. Scire Jr. Many rate constant measurements, including some "direct" measurements, involve fitting a complex reaction mechanism to experimental data. Two techniques for estimating the error in such measurements were compared. In the first technique, local first-order elementary sensitivities were used to rapidly estimate the sensitivity of the fitted rate constants to the remaining mechanism parameters. Our group and others have used this technique for error estimation and experimental design. However, the nonlinearity and strong coupling found in reaction mechanisms make verification against globally valid results desirable. Here, the local results were compared with analogous importance-sampled Monte Carlo calculations in which the parameter values were distributed according to their uncertainties. Two of our published rate measurements were examined. The local uncertainty estimates were compared with Monte Carlo confidence intervals. The local sensitivity coefficients were compared with coefficients from first and second-degree polynomial regressions over the whole parameter space. The first-order uncertainty estimates were found to be sufficiently accurate for experimental design, but were subject to error in the presence of higher order sensitivities. In addition, global uncertainty estimates were found to narrow when the quality of the fit was used to weight the randomly distributed points. For final results, the global technique was found to provide efficient, accurate values without the assumptions inherent in the local analysis. The rigorous error estimates derived in this way were used to address literature criticism of one of the studies discussed here. Given its efficiency and the variety of problems it can detect, the global technique could also be used to check local results during the experimental design phase. The global routine, coded using SENKIN, can easily be extended to different types of data, and therefore can serve as a valuable tool for assessing error in rate constants determined using complex mechanisms. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 784,802, 2001 [source] Low and high order light scattering in particulate mediaLASER PHYSICS LETTERS, Issue 8 2004I. V. Meglinski Abstract We present the results of a theoretical study providing details of propagation of laser radiation within disperse randomly inhomogeneous intermediately single-to-multiple scattering media. A quantitative analysis of scattering orders in the transition from single to multiple scattering is presented. Crossed source-detector fiber optics geometry used to separate the intensity of single scattering from higher scattering orders. The results demonstrate good agreement between analytical and Monte Carlo techniques. This validates the use of the Monte Carlo approach in the intermediate single-to-multiple scattering regime. The method used can be applied to verify analytical results against experiment via the Monte Carlo calculations that include imperfections of the experiment. (© 2004 by ASTRO, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Low frequency spin dynamics in the quantum magnet copper pyrazine dinitratePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2010H. Kühne Abstract The S,=,1/2 antiferromagnetic Heisenberg chain exhibits a magnetic field driven quantum critical point. We study the low frequency spin dynamics in copper pyrazine dinitrate (CuPzN), a realization of this model system of quantum magnetism, by means of 13C-NMR spectroscopy. Measurements of the nuclear spin,lattice relaxation rate in the vicinity of the saturation field are compared with quantum Monte Carlo calculations of the dynamic structure factor. Both show a strong divergence of low energy excitations at temperatures in the quantum regime. The analysis of the anisotropic -rates and frequency shifts allows one to disentangle the contributions from transverse and longitudinal spin fluctuations for a selective study and to determine the transfer of delocalized spin moments from copper to the neighboring nitrogen atoms. [source] Hydrogen Adsorption and Diffusion in p - tert -Butylcalix[4]arene: An Experimental and Molecular Simulation StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2010Dr. Saman Alavi Abstract Experimental adsorption isotherms were measured and computer simulations were performed to determine the nature of the H2 gas uptake in the low-density p - tert -butylcalix[4]arene (tBC) phase. 1H,NMR peak intensity measurements for pressures up to 175,bar were used to determine the H2 adsorption isotherm. Weak surface adsorption (up to ,2,mass,% H2) and stronger adsorption (not exceeding 0.25,mass,% or one H2 per calixarene bowl) inside the calixarene phase were detected. The latter type of adsorbed H2 molecule has restricted motion and shows a reversible gas adsorption/desorption cycle. Pulsed field gradient (PFG) NMR pressurization/depressurization measurements were performed to study the diffusion of H2 in the calixarene phases. Direct adsorption isotherms by exposure of the calixarene phase to pressures of H2 gas to ,60,bar are also presented, and show a maximum H2 adsorption of 0.4,H2 per calixarene bowl. Adsorption isotherms of H2 in bulk tBC have been simulated using grand canonical Monte Carlo calculations in a rigid tBC framework, and yield adsorptions of ,1,H2 per calixarene bowl at saturation. Classical molecular dynamics simulations with a fully flexible calixarene molecular force field are used to determine the guest distribution and inclusion energy of the H2 in the solid with different loadings. [source] | |||||||||||||