An improved Monte Carlo method for direct calculation of the density of states

We present an efficient Monte Carlo algorithm for determining the density of states which is based on the statistics of transition probabilities between states. By measuring the infinite temperature transition probabilities--that is, the probabilities associated with move proposal only--we are able to extract excellent estimates of the density of states. When this estimator is used in conjunction with a Wang-Landau sampling scheme [F. Wang and D. P. Landau, Phys. Rev. Lett. 86, 2050 (2001)], we quickly achieve uniform sampling of macrostates (e.g., energies) and systematically refine the calculated density of states. This approach requires only potential energy evaluations, continues to improve the statistical quality of its results as the simulation time is extended, and is applicable to both lattice and continuum systems. We test the algorithm on the Lennard-Jones liquid and demonstrate good statistical convergence properties.Comment: 7 pages, 4 figures. to appear in Journal of Chemical Physic

Exploring the democratic potential of online social networking: The scope and limitations of e-participation

Copyright © 2012 by the Association for Information Systems.The availability and promise of social networking technologies with their perceived open philosophy has increasingly inspired citizens around the world to participate in political activity on the Web. Recent examples range from opposing public policies, such as government funding cuts, to organizing revolutionary social movements, such as those in the Middle East and North Africa. Although online spaces create remarkable opportunities for various forms of political action, there are concerns over the power of existing institutions to control and even censor such interaction spaces. The objective of this article is to draw together different insights on the online engagement phenomenon, highlighting both its potential and limitations as a mechanism for fostering democratic debate and influencing policy making. We examine recent examples from Europe, the Middle East and Latin America. Finally, we summarize the implications of our work and outline directions for further research

We have always been transreligious: An introduction to transreligiosity

The paper is a proposition and exploration of the term ‘transreligiosity’. We argue that transreligiosity is more apt to describe the transgressive character of religiosity, focusing more particularly on the transversality of spaces, symbolic or otherwise, which are created in religious phenomena. We examine the porosity of religious boundaries and, ultimately, propose the term transreligiosity to embrace them, placing emphasis on their transreligious character, while perceiving them as a pivotal fragment of transreligiosity. We take some of Latour’s key concepts on ‘purification’, to argue for the ultimate impossibility of it in the sphere of religiosity. While processes of purification have been powerful through efforts to institutionalize and centralize religiosity, on a vernacular level, this has had a contrary effect. Religious subjects have been distanced from a more direct participation (‘mediation’). Hence, they are constantly creating transreligious instances in order to abolish and transgress those rigid borders.info:eu-repo/semantics/acceptedVersio

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

Crowding of Polymer Coils and Demixing in Nanoparticle-Polymer Mixtures

The Asakura-Oosawa-Vrij (AOV) model of colloid-polymer mixtures idealizes nonadsorbing polymers as effective spheres that are fixed in size and impenetrable to hard particles. Real polymer coils, however, are intrinsically polydisperse in size (radius of gyration) and may be penetrated by smaller particles. Crowding by nanoparticles can affect the size distribution of polymer coils, thereby modifying effective depletion interactions and thermodynamic stability. To analyse the influence of crowding on polymer conformations and demixing phase behaviour, we adapt the AOV model to mixtures of nanoparticles and ideal, penetrable polymer coils that can vary in size. We perform Gibbs ensemble Monte Carlo simulations, including trial nanoparticle-polymer overlaps and variations in radius of gyration. Results are compared with predictions of free-volume theory. Simulation and theory consistently predict that ideal polymers are compressed by nanoparticles and that compressibility and penetrability stabilise nanoparticle-polymer mixtures.Comment: 18 pages, 4 figure

Phase diagrams in the lattice RPM model: from order-disorder to gas-liquid phase transition

The phase behavior of the lattice restricted primitive model (RPM) for ionic systems with additional short-range nearest neighbor (nn) repulsive interactions has been studied by grand canonical Monte Carlo simulations. We obtain a rich phase behavior as the nn strength is varied. In particular, the phase diagram is very similar to the continuum RPM model for high nn strength. Specifically, we have found both gas-liquid phase separation, with associated Ising critical point, and first-order liquid-solid transition. We discuss how the line of continuous order-disorder transitions present for the low nn strength changes into the continuum-space behavior as one increases the nn strength and compare our findings with recent theoretical results by Ciach and Stell [Phys. Rev. Lett. {\bf 91}, 060601 (2003)].Comment: 7 pages, 10 figure

Thermodynamics of Electrolytes on Anisotropic Lattices

The phase behavior of ionic fluids on simple cubic and tetragonal (anisotropic) lattices has been studied by grand canonical Monte Carlo simulations. Systems with both the true lattice Coulombic potential and continuous-space $1/r$ electrostatic interactions have been investigated. At all degrees of anisotropy, only coexistence between a disordered low-density phase and an ordered high-density phase with the structure similar to ionic crystal was found, in contrast to recent theoretical predictions. Tricritical parameters were determined to be monotonously increasing functions of anisotropy parameters which is consistent with theoretical calculations based on the Debye-H\"uckel approach. At large anisotropies a two-dimensional-like behavior is observed, from which we estimated the dimensionless tricritical temperature and density for the two-dimensional square lattice electrolyte to be $T^*_{tri}=0.14$ and $\rho^*_{tri} = 0.70$.Comment: submitted to PR

Extreme Events in Resonant Radiation from Three-dimensional Light Bullets

We report measurements that show extreme events in the statistics of resonant radiation emitted from spatiotemporal light bullets. We trace the origin of these extreme events back to instabilities leading to steep gradients in the temporal profile of the intense light bullet that occur during the initial collapse dynamics. Numerical simulations reproduce the extreme valued statistics of the resonant radiation which are found to be intrinsically linked to the simultaneous occurrence of both temporal and spatial self-focusing dynamics. Small fluctuations in both the input energy and in the spatial phase curvature explain the observed extreme behaviour.Comment: 5 pages, 5 figures, submitte

Structural and magnetic properties of Ru/Ni multilayers

International audienceRu/Ni multilayers of different Ni thickness have been fabricated using magnetron sputtering. The structure of the multilayers has been determined by grazing incidence X-ray diffraction and X-ray reflectivity and their magnetic properties by magnetization and polarized neutron reflectivity measurements. The presence of Ru leads to the formation of a hexagonal Ni structure within interfacial layer ~1 nm above each Ru layer, while the rest of the Ni layer relaxes to the equilibrium fcc structure. The hcp Ni interfacial layer has a substantially increased cell volume is ferromagnetic with an atomic magnetic moment that increases with Ni layer thickness but remains lower than the value predicted from ab initio calculations