99,325 research outputs found
Computer Simulations of Quantum Chains
We report recent progress in computer simulations of quantum systems
described in the path-integral formulation. For the example of the
quantum chain we show that the accuracy of the simulation may greatly be
enhanced by a combination of multigrid update techniques with a refined
discretization scheme. This allows us to assess the accuracy of a variational
approximation.Comment: 5 pages, LaTeX + 2 postscript figures. Talk presented by TS at "Path
Integrals from meV to MeV: Dubna '96". See also
http://www.cond-mat.physik.uni-mainz.de/~janke/doc/home_janke.htm
Evolutionary Games and Computer Simulations
The prisoner's dilemma has long been considered the paradigm for studying the
emergence of cooperation among selfish individuals. Because of its importance,
it has been studied through computer experiments as well as in the laboratory
and by analytical means. However, there are important differences between the
way a system composed of many interacting elements is simulated by a digital
machine and the manner in which it behaves when studied in real experiments. In
some instances, these disparities can be marked enough so as to cast doubt on
the implications of cellular automata type simulations for the study of
cooperation in social systems. In particular, if such a simulation imposes
space-time granularity, then its ability to describe the real world may be
compromised. Indeed, we show that the results of digital simulations regarding
territoriality and cooperation differ greatly when time is discrete as opposed
to continuous.Comment: 8 pages. Also available through anonymous ftp from parcftp.xerox.com
in the directory /pub/dynamics as pdilemma.p
A practical guide to computer simulations
Here practical aspects of conducting research via computer simulations are
discussed. The following issues are addressed: software engineering,
object-oriented software development, programming style, macros, make files,
scripts, libraries, random numbers, testing, debugging, data plotting, curve
fitting, finite-size scaling, information retrieval, and preparing
presentations.
Because of the limited space, usually only short introductions to the
specific areas are given and references to more extensive literature are cited.
All examples of code are in C/C++.Comment: 69 pages, with permission of Wiley-VCH, see http://www.wiley-vch.de
(some screenshots with poor quality due to arXiv size restrictions) A
comprehensively extended version will appear in spring 2009 as book at
Word-Scientific, see http://www.worldscibooks.com/physics/6988.htm
Computer Simulations of Supercooled Liquids and Glasses
After a brief introduction to the dynamics of supercooled liquids, we discuss
some of the advantages and drawbacks of computer simulations of such systems.
Subsequently we present the results of computer simulations in which the
dynamics of a fragile glass former, a binary Lennard-Jones system, is compared
to the one of a strong glass former, SiO_2. This comparison gives evidence that
the reason for the different temperature dependence of these two types of glass
formers lies in the transport mechanism for the particles in the vicinity of
T_c, the critical temperature of mode-coupling theory. Whereas the one of the
fragile glass former is described very well by the ideal version of
mode-coupling theory, the one for the strong glass former is dominated by
activated processes. In the last part of the article we review some simulations
of glass formers in which the dynamics below the glass transition temperature
was investigated. We show that such simulations might help to establish a
connection between systems with self generated disorder (e.g. structural
glasses) and quenched disorder (e.g. spin glasses).Comment: 37 pages of Latex, 11 figures, to appear as a Topical Review article
in J. Phys.: Condens. Matte
Implicit Finite-Size Effects in Computer Simulations
The influence of periodic boundary conditions (implicit finite-size effects)
on the anisotropy of pair correlations in computer simulations is studied for a
dense classical fluid of pair-wise interacting krypton atoms near the triple
point. Molecular dynamics simulation data for the pair distribution function of
N-particle systems, as a function of radial distance, polar angle, and
azimuthal angle are compared directly with corresponding theoretical
predictions [L. R. Pratt and S. W. Haan, J. Chem. Phys. 74, 1864 (1981)]. For
relatively small systems of N=60, 80, and 108 atoms, significant angular
variation is observed, which is qualitatively, and in several cases
quantitatively, well predicted by theory. Finite-size corrections to the
spherically-averaged radial distribution function, however, are found to be
comparable to random statistical errors for runs of 10^5 time steps.Comment: plain TeX, 14 pages + 16 postscript figures, to appear Z. Phys.
Computer simulations of hard pear-shaped particles
We report results obtained from Monte Carlo simulations investi-
gating mesophase formation in two model systems of hard pear-shaped
particles. The first model considered is a hard variant of the trun-
cated Stone-Expansion model previously shown to form nematic and
smectic mesophases when embedded within a 12-6 Gay-Berne-like po-
tential [1]. When stripped of its attractive interactions, however, this system is found to lose its liquid crystalline phases. For particles of length to breadth ratio k = 3, glassy behaviour is seen at high pressures, whereas for k = 5 several bi-layer-like domains are seen, with high intradomain order but little interdomain orientational correlation. For the second model, which uses a parametric shape parameter based on the generalised Gay-Berne formalism, results are presented for particles with elongation k = 3; 4 and 5. Here, the systems with k = 3 and 4 fail to display orientationally ordered phases, but that with k = 5 shows isotropic, nematic and, unusually for a hard-particle model, interdigitated smectic A2 phases.</p
- …