349 research outputs found
Three-dimensional Ising model in the fixed-magnetization ensemble: a Monte Carlo study
We study the three-dimensional Ising model at the critical point in the
fixed-magnetization ensemble, by means of the recently developed geometric
cluster Monte Carlo algorithm. We define a magnetic-field-like quantity in
terms of microscopic spin-up and spin-down probabilities in a given
configuration of neighbors. In the thermodynamic limit, the relation between
this field and the magnetization reduces to the canonical relation M(h).
However, for finite systems, the relation is different. We establish a close
connection between this relation and the probability distribution of the
magnetization of a finite-size system in the canonical ensemble.Comment: 8 pages, 2 Postscript figures, uses RevTe
RNA structure prediction from evolutionary patterns of nucleotide composition
Structural elements in RNA molecules have a distinct nucleotide composition, which changes gradually over evolutionary time. We discovered certain features of these compositional patterns that are shared between all RNA families. Based on this information, we developed a structure prediction method that evaluates candidate structures for a set of homologous RNAs on their ability to reproduce the patterns exhibited by biological structures. The method is named SPuNC for âStructure Prediction using Nucleotide Compositionâ. In a performance test on a diverse set of RNA families we demonstrate that the SPuNC algorithm succeeds in selecting the most realistic structures in an ensemble. The average accuracy of top-scoring structures is significantly higher than the average accuracy of all ensemble members (improvements of more than 20% observed). In addition, a consensus structure that includes the most reliable base pairs gleaned from a set of top-scoring structures is generally more accurate than a consensus derived from the full structural ensemble. Our method achieves better accuracy than existing methods on several RNA families, including novel riboswitches and ribozymes. The results clearly show that nucleotide composition can be used to reveal the quality of RNA structures and thus the presented technique should be added to the set of prediction tools
Graphical representations and cluster algorithms for critical points with fields
A two-replica graphical representation and associated cluster algorithm is
described that is applicable to ferromagnetic Ising systems with arbitrary
fields. Critical points are associated with the percolation threshold of the
graphical representation. Results from numerical simulations of the Ising model
in a staggered field are presented. The dynamic exponent for the algorithm is
measured to be less than 0.5.Comment: Revtex, 12 pages with 2 figure
Generalized Geometric Cluster Algorithm for Fluid Simulation
We present a detailed description of the generalized geometric cluster
algorithm for the efficient simulation of continuum fluids. The connection with
well-known cluster algorithms for lattice spin models is discussed, and an
explicit full cluster decomposition is derived for a particle configuration in
a fluid. We investigate a number of basic properties of the geometric cluster
algorithm, including the dependence of the cluster-size distribution on density
and temperature. Practical aspects of its implementation and possible
extensions are discussed. The capabilities and efficiency of our approach are
illustrated by means of two example studies.Comment: Accepted for publication in Phys. Rev. E. Follow-up to
cond-mat/041274
Numerical Solution of Hard-Core Mixtures
We study the equilibrium phase diagram of binary mixtures of hard spheres as
well as of parallel hard cubes. A superior cluster algorithm allows us to
establish and to access the demixed phase for both systems and to investigate
the subtle interplay between short-range depletion and long-range demixing.Comment: 4 pages, 2 figure
Physical tests for Random Numbers in Simulations
We propose three physical tests to measure correlations in random numbers
used in Monte Carlo simulations. The first test uses autocorrelation times of
certain physical quantities when the Ising model is simulated with the Wolff
algorithm. The second test is based on random walks, and the third on blocks of
n successive numbers. We apply the tests to show that recent errors in high
precision simulations using generalized feedback shift register algorithms are
due to short range correlations in random number sequences. We also determine
the length of these correlations.Comment: 16 pages, Post Script file, HU-TFT-94-
Investigations of primary and secondary particulate matter of different wood combustion appliances with a high-resolution time-of-flight aerosol mass spectrometer
A series of photo-oxidation smog chamber experiments were performed to investigate the primary emissions and secondary aerosol formation from two different log wood burners and a residential pellet burner under different burning conditions: starting and flaming phase. Emissions were sampled from the chimney and injected into the smog chamber leading to primary organic aerosol (POA) concentrations comparable to ambient levels. The composition of the aerosol was measured by an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS) and black carbon (BC) instrumentation. The primary emissions were then exposed to xenon light to initiate photo-chemistry and subsequent secondary organic aerosol (SOA) production. After correcting for wall losses, the average increase in organic matter (OM) concentrations by SOA formation for the starting and flaming phase experiments with the two log wood burners was found to be a factor of 4.1&plusmn;1.4 after five hours of aging. No SOA formation was observed for the stable burning phase of the pellet burner. The startup emissions of the pellet burner showed an increase in OM concentration by a factor of 3.3. Including the measured SOA formation potential, average emission factors of BC+POA+SOA, calculated from CO<sub>2</sub> emission, were found to be in the range of 0.04 to 3.9 g/kg wood for the stable burning pellet burner and an old log wood burner during startup respectively. SOA contributed significantly to the ion C<sub>2</sub>H<sub>4</sub>O<sub>2</sub><sup>+</sup> at mass to charge ratio <i>m/z</i> 60, a commonly used marker for primary emissions of wood burning. This contribution at <i>m/z</i> 60 can overcompensate for the degradation of levoglucosan leading to an overestimation of the contribution of wood burning or biomass burning to the total OM. The primary organic emissions from the three different burners showed a wide range in O:C atomic ratio (0.19&minus;0.60) for the starting and flaming conditions, which also increased during aging. Primary wood burning emissions have a rather low relative contribution at <i>m/z</i> 43 (<i>f</i> 43) to the total organic mass spectrum. The non-oxidized fragment C<sub>3</sub>H<sub>7</sub><sup>+</sup> has a considerable contribution at <i>m/z</i> 43 for the fresh OA with an increasing contribution of the oxygenated ion C<sub>2</sub>H<sub>3</sub>O<sup>+</sup> during aging. After five hours of aging, the OA has a rather low C<sub>2</sub>H<sub>3</sub>O<sup>+</sup> signal for a given CO<sub>2</sub><sup>+</sup> fraction, possibly indicating a higher ratio of acid to non-acid oxygenated compounds in wood burning OA compared to other oxygenated organic aerosol (OOA)
Monte Carlo Renormalization of the 3-D Ising model: Analyticity and Convergence
We review the assumptions on which the Monte Carlo renormalization technique
is based, in particular the analyticity of the block spin transformations. On
this basis, we select an optimized Kadanoff blocking rule in combination with
the simulation of a d=3 Ising model with reduced corrections to scaling. This
is achieved by including interactions with second and third neighbors. As a
consequence of the improved analyticity properties, this Monte Carlo
renormalization method yields a fast convergence and a high accuracy. The
results for the critical exponents are y_H=2.481(1) and y_T=1.585(3).Comment: RevTeX, 4 PostScript file
A thermodynamically self-consistent theory for the Blume-Capel model
We use a self-consistent Ornstein-Zernike approximation to study the
Blume-Capel ferromagnet on three-dimensional lattices. The correlation
functions and the thermodynamics are obtained from the solution of two coupled
partial differential equations. The theory provides a comprehensive and
accurate description of the phase diagram in all regions, including the wing
boundaries in non-zero magnetic field. In particular, the coordinates of the
tricritical point are in very good agreement with the best estimates from
simulation or series expansion. Numerical and analytical analysis strongly
suggest that the theory predicts a universal Ising-like critical behavior along
the -line and the wing critical lines, and a tricritical behavior
governed by mean-field exponents.Comment: 11 figures. to appear in Physical Review
Retinal Biomarker Discovery for Dementia in an Elderly Diabetic Population
Dementia is a devastating disease, and has severe implications on affected individuals, their family and wider society. A growing body of literature is studying the association of retinal microvasculature measurement with dementia. We present a pilot study testing the strength of groups of conventional (semantic) and texture-based (non-semantic) measurements extracted from retinal fundus camera images to classify patients with and without dementia. We performed a 500-trial bootstrap analysis with regularized logistic regression on a cohort of 1,742 elderly diabetic individuals (median age 72.2). Age was the strongest predictor for this elderly cohort. Semantic retinal measurements featured in up to 81% of the bootstrap trials, with arterial caliber and optic disk size chosen most often, suggesting that they do complement age when selected together in a classifier. Textural features were able to train classifiers that match the performance of age, suggesting they are potentially a rich source of information for dementia outcome classification
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