410 research outputs found
On the Wang-Landau Method for Off-Lattice Simulations in the "Uniform" Ensemble
We present a rigorous derivation for off-lattice implementations of the
so-called "random-walk" algorithm recently introduced by Wang and Landau [PRL
86, 2050 (2001)]. Originally developed for discrete systems, the algorithm
samples configurations according to their inverse density of states using
Monte-Carlo moves; the estimate for the density of states is refined at each
simulation step and is ultimately used to calculate thermodynamic properties.
We present an implementation for atomic systems based on a rigorous separation
of kinetic and configurational contributions to the density of states. By
constructing a "uniform" ensemble for configurational degrees of freedom--in
which all potential energies, volumes, and numbers of particles are equally
probable--we establish a framework for the correct implementation of simulation
acceptance criteria and calculation of thermodynamic averages in the continuum
case. To demonstrate the generality of our approach, we perform sample
calculations for the Lennard-Jones fluid using two implementation variants and
in both cases find good agreement with established literature values for the
vapor-liquid coexistence locus.Comment: 21 pages, 4 figure
Sine-Gordon mean field theory of a Coulomb Gas
Sine-Gordon field theory is used to investigate the phase diagram of a
neutral Coulomb gas. A variational mean field free energy is constructed and
the corresponding phase diagrams in two (2d) and three dimensions (3d) are
obtained. When analyzed in terms of chemical potential, the Sine-Gordon theory
predicts the phase diagram topologically identical with the Monte Carlo
simulations and a recently developed Debye-H\"uckel-Bjerrum (DHBj) theory. In
2d we find that the infinite order Kosterlitz-Thouless line terminates in a
tricritical point, after which the metal-insulator transition becomes first
order. However, when the transformation from chemical potential to the density
is made the whole of the insulating phase is mapped onto zero density.Comment: 5 pages, Revtex with twocolumn style, 2 Postscript figures. Submitted
to PR
Criticality in confined ionic fluids
A theory of a confined two dimensional electrolyte is presented. The positive
and negative ions, interacting by a potential, are constrained to move on
an interface separating two solvents with dielectric constants and
. It is shown that the Debye-H\"uckel type of theory predicts that
the this 2d Coulomb fluid should undergo a phase separation into a coexisting
liquid (high density) and gas (low density) phases. We argue, however, that the
formation of polymer-like chains of alternating positive and negative ions can
prevent this phase transition from taking place.Comment: RevTex, no figures, in press Phys. Rev.
Sciatica due to extrapelvic heterotopic ossification: A case report
<p>Abstract</p> <p>Introduction</p> <p>Sciatica is a common problem, usually caused by disc herniation or spinal stenosis. Low back pain is also present in most cases. When sciatica is the unique clinical finding, especially in young patients, extraspinal pathology should be investigated.</p> <p>Case presentation</p> <p>We describe a rare case of sciatica in a 32-year-old man, which was developed as a complication of post-traumatic pelvic heterotopic ossification. During the operation, the sciatic nerve was found to be bluish, distorted and compressed in an hourglass fashion around a heterotopic bone mass. The heterotopic bone tissue, 4 cm in diameter, was removed and the patient had fully recovered 3 months after the operation.</p> <p>Conclusion</p> <p>In cases of sciatica without back pain, the possibility of direct pressure of the sciatic nerve from cysts, tumours or bone, as in the present case, should be considered.</p
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