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1.
Diffusion-limited cluster aggregation and gelation of hard spheres is simulated using off-lattice Monte Carlo simulations.
A comparison is made of the wall-particle correlation function with the particle-particle correlation function over a range
of volume fractions, both for the initial system of randomly distributed spheres and for the final gel state. For randomly
distributed spheres the correlation functions are compared with theoretical results using the Ornstein-Zernike equation and
the Percus-Yevick closure. At high volume fractions (φ > 40%) gelation has little influence on the correlation function, but
for φ < 10% it is a universal function of the distance normalized by correlation length (ξ) of the bulk. The width of the
depletion layer is about 0.5ξ. The concentration increases as a power law from the wall up to r ≈ ξ, where it reaches a weak maximum before decreasing to the bulk value. 相似文献
2.
Babu S Rottereau M Nicolai T Gimel JC Durand D 《The European physical journal. E, Soft matter》2006,19(2):203-211
Off-lattice dynamic Monte-Carlo simulations were done of reversible cluster-cluster aggregation for spheres that form rigid
bonds at contact. The equilibrium properties were found to be determined by the life time of encounters between two particles
(te). te is a function not only of the probability to form or break a bond, but also of the elementary step size of the Brownian motion
of the particles. In the flocculation regime the fractal dimension of the clusters is df=2.0 and the size distribution has a power law decay with exponent τ=1.5. At larger values of te transient gels are formed. Close to the
percolation threshold the clusters have a fractal dimension df=2.7 and the power law exponent of the size distribution is τ=2.1. The transition between flocculation and percolation
occurs at a characteristic weight average aggregation number that decreases with increasing volume fraction. 相似文献
3.
Irreversible diffusion limited cluster aggregation (DLCA) of hard spheres was simulated using Brownian cluster dynamics. Bound
spheres were allowed to move freely within a specified range, but no bond breaking was allowed. The structure and size distribution
of the clusters was investigated before gelation. The pair correlation function and the static structure factor of the gels
were determined as a function of the volume fraction and time. Slippery bonds led to local densification of the clusters and
the gels, with a certain degree of order. At low volume fractions densification of the clusters occurred during their growth,
but at higher volume fractions it occurred mainly after gelation. At very low volume fractions, the large-scale structure
(fractal dimension), size distribution and growth kinetics of the clusters was found to be close to that known for DLCA with
rigid bonds. Restructuring of the gels continued for long times, indicating that aging processes in systems with strong attraction
do not necessarily involve bond breaking. The mean-square displacement of particles in the gels was determined. It is shown
to be highly heterogeneous and to increase with decreasing volume fraction. 相似文献
4.
Koga T 《The European physical journal. E, Soft matter》2005,17(3):381-388
The formation of intramolecular micelles in copolymers with periodic sequence, where hydrophobic units (stickers) are periodically placed along the chain, is studied by using multicanonical Monte Carlo computer simulations for an off-lattice bead-rod model in three dimensions. With decreasing the temperature, a transition from random-coil conformations to micelles occurs and flower-type micelles are formed via the transition. The number of stickers forming a micelle core is limited by the excluded-volume effect of loop chains around micelle cores. By this effect, two intramolecular micelles are formed for long polymer chains with 60 bonds via the coil-to-micelle transition. By further decreasing the temperature, we find that another transition, i.e., a micelle-to-micelle transition, takes place. At this transition point, the two intramolecular micelles merge into one micelle. Furthermore, we extend the multicanonical MC method to study elastic properties of single polymer chains with strong attractive interactions under external force fields, and study how the intramolecular micellization affects the elastic property of single polymer chains. 相似文献
5.
A. Hacura M. Jadamus-Hacura A. Kocot 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(4):551-553
This work has been prepared for the purpose of presenting the methodology and uses of the Monte Carlo simulation technique
as applied in the evaluation of investment projects to analyze and assess risk. In the deterministic appraisal the basic decision
rule for a project is simply to accept or reject the project depending on whether its net present value (NPV) is positive
or negative, respectively. Similarly, when choosing among alternative (mutually exclusive) projects, the decision rule is
to select the one with the highest NPV, provided that it is positive. Recognizing the fact that the key project variables
(as: volume of sales, sales price, costs) are not certain, an appraisal report is usually supplemented to include sensitivity
and scenario analysis tests. Both tests are static and rather arbitrary in their nature. During the simulation process, random
scenarios are built up using input values for the project's key uncertain variables, which are selected from appropriate probability
distributions. The results are collected and analyzed statistically so as to arrive at a probability distribution of the potential
outcomes of the project and to estimate various measures of project risk.
Received 25 September 2000 相似文献
6.
J. Ferkinghoff-Borg 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(3):481-484
We present a new, maximum-likelihood based method to combine data from a multiple number of Monte Carlo simulations performed
within any type of ensemble. The method offers an efficient iterative scheme to obtain the density of states of a wide range
of energies as well as of other macroscopic variables. It should in particular be useful for the study of systems with a rough
energy landscape.
Received 4 June 2002 Published online 14 October 2002
RID="a"
ID="a"e-mail: borg@alf.nbi.dk 相似文献
7.
R. Obermaier 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(3):407-409
Comment on Eur. Phys. J. B 20, 551 (2001)
Since Hertz major work on investment appraisal using the Monte Carlo Simulation technique, the so called “Risk Analysis”
has become a standard tool for supporting investment decisions [1,2]. A main problem in investment appraisal is to consider
and specify the risk of investment projects in an appropriate way, for enabling consistent project evaluation. In calculating
a risky project's net present value (NPV) the major difficulty is to quantify the project's risk for quantifying an appropriate
risk adjusted discount rate (RADR). Theoretically not founded risk adjusted discount rates face a lot of critique. Furthermore
it is discussed that the incorporation of a constant risk factor into the discount rate makes a certain assumption about the
resolution of uncertainty over time [3] and finally that a single net present value could not in general reflect risk properly.
Especially in consequence of the last point the proponents of simulation argue that a whole distribution of net present values
shows a project's risk better than a single number. In the special issue “Econophysics” of this journal Hacura et al. tried to describe the methodology and use of Monte Carlo Simulation in investment appraisal [4]. The purpose of this comment
is to point out three fundamental flaws in that article.
Received 29 April 2002 Published online 19 December 2002
RID="a"
ID="a"e-mail: robert.obermaier@wiwi.uni-regensburg.de 相似文献
8.
J. Kaupužs R. V.N. Melnik J. Rimshans 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):363-370
Monte Carlo simulations of magnetization and susceptibility in the 3D XY
model are performed for system sizes up to L=384 (significantly exceeding the
largest size L=160 considered in work published previously), and fields h
≥ 0.0003125 at two different coupling constants β=0.5, and
β=0.55 in the ordered phase. We examine the prediction of the standard
theory that the longitudinal susceptibility χ∥ has a Goldstone mode singularity
such that χ∥ ∝h-1/2 holds when h↦0. Most
of our results, however, support another theoretical prediction that the
singularity is of a more general form χ∥ ∝hρ-1,
where 1/2<ρ<1 is a universal exponent related to the ∼hρ
variation of the magnetization. 相似文献
9.
M. Ercsey-Ravasz T. Roska Z. Néda 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(3):407-411
The computational paradigm represented by Cellular Neural/nonlinear Networks (CNN) and the CNN Universal Machine (CNN-UM)
as a Cellular Wave Computer, gives new perspectives for computational physics.
Many numerical problems and simulations can be elegantly
addressed on this fully parallelized and analogic architecture. Here we study the possibility of performing
stochastic simulations on this chip. First a realistic random number generator is implemented on the
CNN-UM, and then as an example the two-dimensional Ising model is studied by Monte Carlo type simulations. The results obtained
on an experimental version of the CNN-UM with 128 ×128 cells are in good agreement with the results
obtained on digital computers. Computational time measurements suggest that the developing trend of
the CNN-UM chips — increasing the lattice size and the number of local logic memories —
will assure an important advantage for the CNN-UM in the near future. 相似文献
10.
The static structure factor (S(q)) of dispersions and gels of disk-like mineral colloids (Laponite) was investigated using time- and ensemble-averaged light
scattering. The evolution of S(q) in time after increasing the ionic strength of well-dispersed Laponite suspensions shows that Laponite aggregates and forms
fractal clusters. The structure of the aggregates does not depend on the ionic strength, but the rate of growth increases
very strongly with the ionic strength. At concentrations below about 3 g/l (0.12% v/v) the aggregates sediment while at higher
concentrations space-filling gels are formed. The gels are homogeneous on length scales larger than the correlation length
which decreases strongly with decreasing ionic strength and increasing concentration. However, the local structure is the
same, independent of the concentration and the ionic strength.
Received 6 August 2000 and Received in final form 16 March 2001 相似文献
11.
E.L. Præstgaard B. Schmittmann R.K.P. Zia 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):675-695
We investigate the collective behavior of an Ising lattice gas, driven to non-equilibrium steady states by being coupled to
two thermal baths. Monte Carlo methods are applied to a two-dimensional system in which one of the baths is fixed at infinite
temperature. Both generic long range correlations in the disordered state and critical properties near the second order transition
are measured. Anisotropic scaling, a key feature near criticality, is used to extract and some critical exponents. On the theoretical front, a continuum theory, in the spirit of Landau-Ginzburg, is presented.
Being a renormalizable theory, its predictions can be computed by standard methods of -expansions and found to be consistent with simulation data. In particular, the critical behavior of this system belongs to
a universality class which is quite different from the uniformly driven Ising model.
Received 4 October 2000 相似文献
12.
J. J. Liétor-Santos C. Kim P. J. Lu A. Fernández-Nieves D. A. Weitz 《The European physical journal. E, Soft matter》2009,28(2):159-164
We study the compression of depletion gels under the influence of a gravitational stress by monitoring the time evolution
of the gel interface and the local volume fraction, , inside the gel. We find is not constant throughout the gel. Instead, there is a volume fraction gradient that develops and grows along the gel height
as the compression process proceeds. Our results are correctly described by a non-linear poroelastic model that explicitly
incorporates the -dependence of the gravitational, elastic and viscous stresses acting on the gel. 相似文献
13.
A. Malakis P. Kalozoumis N. Tyraskis 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,50(1-2):63-67
We apply a new entropic scheme to study the critical behavior of
the square-lattice Ising model with nearest- and
next-nearest-neighbor antiferromagnetic interactions. Estimates of
the present scheme are compared with those of the Metropolis
algorithm. We consider interactions in the range where
superantiferromagnetic (SAF) order appears at low temperatures. A
recent prediction of a first-order transition along a certain
range (0.5–1.2) of the interaction ratio (R=Jnnn/Jnn) is
examined by generating accurate data for large lattices at a
particular value of the ratio (R=1). Our study does not support
a first-order transition and a convincing finite-size scaling
analysis of the model is presented, yielding accurate estimates
for all critical exponents for R=1. The magnetic exponents are
found to obey “weak universality” in accordance with a previous
conjecture. 相似文献
14.
R. V. Chamberlin G. H. Wolf 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,67(4):495-499
The entropy per particle in most Monte-Carlo simulations is size dependent
due to correlated energy fluctuations. Guided by nanothermodynamics, we find
a constraint for the Ising model that enhances the fluctuations and lowers
the free energy, while making the entropy homogeneous, additive, and
extensive. Although the average interaction energy becomes size dependent,
the resulting distribution of energies provides a mechanism for the
heterogeneity found in the dynamics of many materials. 相似文献
15.
D. Truzzolillo F. Bordi F. Sciortino C. Cametti 《The European physical journal. E, Soft matter》2009,29(2):229-237
Polymer chains adsorbed onto oppositely charged colloidal particles can significantly modify the particle-particle interactions.
For sufficient amounts of added polymers, the original electrostatic repulsion can even turn into an effective attraction
and relatively large aggregates can form. The attractive interaction contribution between two particles arises from the correlated adsorption of polyions at the oppositely charged particle surfaces, resulting in a non-homogeneous surface charge distribution.
Here, we investigate the aggregation kinetics of polyion-induced colloidal complexes through Monte Carlo simulation, in which
the effect of charge anisotropy is taken into account by a DLVO-like inter-particle potential, as recentely proposed by Velegol
and Thwar (Langmuir 17, 7687 (2001)). The results reveal that the aggregation process slows down due to the progressive increase of the potential
barrier height upon clustering. Within this framework, the experimentally observed cluster phases in polyelectrolyte-liposome
solutions can be interpreted as a kinetic arrested state. 相似文献
16.
M. Janda V. Martišovitš M. Morvová Z. Machala K. Hensel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,45(2):309-315
Motivated by experimental investigations of electrical discharges in N2/CO2/H2O, Monte Carlo (MC) electron dynamics simulations in atmospheric
N2/CO2 mixtures were performed. The goal was to obtain electron energy distribution functions (EEDFs), mean free path, drift velocity,
collision frequency and mean energy of electrons,
rate coefficients of electron-impact reactions, ionisation and attachment coefficients, as functions of the reduced electric
field strength (E/N) and of the concentration of individual gas components. The results obtained by MC simulations were fitted
with polynomials of up to the 3rd order with reasonable accuracy for E/N above 80 Td.
The studied parameters below 80 Td were strongly non-linear as functions of E/N. This is mostly due to the influence of elastic
collisions of electrons with CO2 molecules prevailing in CO2-dominant mixtures for E/N < 40 Td, and vibrational excitation collisions of N2 species prevailing in N2-dominant mixtures for E/N from 40 to 80 Td. The effect of these electron-impact processes was specific for each of the studied
parameters. 相似文献
17.
H. Ahlers A. Engel 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(3):357-364
Building on a variant of the Jarzynski equation we propose a new
method to numerically determine the prior-predictive value in a
Bayesian inference problem. The method generalizes thermodynamic
integration and is not hampered by equilibration problems. We
demonstrate its operation by applying it to two simple examples and
elucidate its performance. In the case of multi-modal posterior
distributions the performance is superior to thermodynamic
integration. 相似文献
18.
G. Puddu 《The European Physical Journal A - Hadrons and Nuclei》2000,9(2):171-181
We extend the recently presented formalism for Monte Carlo calculations of the partition function, for both even and odd particle
number systems (Phys. Rev. C 59, 2500 (1999)), to the calculation of many-body matrix elements of the type <ψ| e
- βℋ|ψ> where |ψ> is a many-body state with a definite angular momentum, parity, neutron and proton numbers. For large β such
matrix elements are dominated by the lowest eigenstate of the many-body Hamiltonian ℋ, corresponding with a given angular
momentum parity and particle number. Emphasis is placed on odd-mass nuclei. Negligible sign fluctuations in the Monte Carlo
calculation are found provided the neutron and proton chemical potentials are properly adjusted. The formalism is applied
to the J
π = 0+ state in 166
Er and to the J
π = 9/2-, 13/2+, 5/2- states in 165
Er using the pairing-plus-quadrupole model.
Received: 28 April 2000 / Accepted: 20 September 2000 相似文献
19.
We study the relevance of including information about rejected Monte-Carlo moves in path-sampling computations of free energies.
For this purpose, we define webs as sets of paths linked by the path-sampling scheme and introduce an associated statistical
ensemble. Within this
web ensemble, we derive and test several statistical averages enabling to include information about configurational and path
quantities belonging to the unselected trial moves.
We numerically observe that retrieving this information does not always result in variance reduction, as theoretically predicted
by Delmas and Jourdain.
To explain the possible detrimental effect of information-retrieving from web sampling, an action for the webs is introduced.
The behaviour of the statistical variance is observed to correlate to an overlapping area of a web action histogram. This
area represents
the probability that a generated web is such that the difference of its action between the targeted and reference ensembles
is lower than the corresponding difference of free energy.
Variance reductions are numerically observed for increased areas, as it is the case for the residence weight method proposed
previously.
More generally, web ensembles yield a rigorous framework for rationalizing existing methods and also for deriving potentially
new methods enabling to retrieve relevant information from rejected trial moves. 相似文献
20.
We propose an extension of the spectroscopic Monte Carlo method to realistic effective interactions. The scheme is applied to the recently introduced GXPF1 interaction for fp nuclei for the ground state of 60Fe, 56Ni, 64Ni and 60Zn. The method hinges on the use of Hartree-Fock-Bogoliubov wave functions (properly projected before variation) and on a reformulation of the effective interaction so that it is a sum of negative squares of Hermitian one-quasi-particle operators, so the application of the Hubbard-Stratonovich transformation to the elementary propagator exp[-
] gives a functional integral over a Hermitian propagator. Limitations and difficulties encountered in the calculation are discussed. 相似文献