Independent Configurations Research Articles

Computational steering in Monte Carlo simulations of thin film polystyrene

High molecular weight polymer systems show very long relaxation times, of the order of milliseconds or more. This time-scale proves practically inaccessible for atomic-scale dynamical simulation such as molecular dynamics. Even with a Monte Carlo (MC) simulation, the generation of statistically independent configurations is non-trivial. Many moves have been proposed to enhance the efficiency of MC simulation of polymers. Each is described by a proposal density Q(x'; x): the probability of selecting the trial state x' given that the system is in the current state x. This proposal density must be parametrized for a particular chain length, chemistry and temperature. Choosing the correct set of parameters can greatly increase the rate at which the system explores its configuration space. Computational steering (CS) provides a new methodology for a systematic search to optimize the proposal densities for individual moves, and to combine groups of moves to greatly improve the equilibration of a model polymer system. We show that monitoring the correlation time of the system is an ideal single parameter for characterizing the efficiency of a proposal density function, and that this is best evaluated by a distributed network of replicas of the system, with the operator making decisions based on the averages generated over these replicas. We have developed an MC code for simulating an anisotropic atomistic bead model which implements the CS paradigm. We report simulations of thin film polystyrene.

Freedom, Community, and Raymond Williams’s Project of a Common Culture

This article is part of a project that proposes and explores an articulation of community, culture, and freedom as the foundation of a cultural studies practice built on the principled intellectual analyses of Raymond Williams and Edward P. Thompson and capable of renewing and developing their democratic project. By focusing on community, culture, and freedom as concepts and practices, this article explores their theoretical status and their political potential today. By constructing a position founded on their articulation, a contemporary mode of intellectual analysis is proposed that can offer alternative views of the world we live in and open independent configurations of the human, the social, and the cultural. In the light of this articulation, the article takes account of recent challenges to cultural studies, reexamines Williams’s common culture project, and reassesses it as a basis for a democratic view and project of common humanity in the light of Zygmunt Bauman’s recent work.

Towards a Miniature Implantable In Vivo Telemetry Monitoring System Dynamically Configurable as a Potentiostat or Galvanostat for Two- and Three-Electrode Biosensors

A miniature implantable and dynamically configurable potentiostat and galvanostat for two- and three-electrode biosensors with a telemetry electronics package was developed to provide remote monitoring of implantable amperometric and voltametric biosensors such as for glucose. Included are circuitry for sensor biasing, a transimpedance amplifier to produce the sensor proportional output signal, and a transceiver (transmitter and receiver) which can both receive setup parameter values and transmit the biosensor concentration data to a corresponding remote transceiver and computer for monitoring. Remotely configurable features included in the in vivo implanted unit include: sensor excitation changes; filter frequency cutoff values, amplifier gain; and transmission intervals utilizing 303.825-MHz UHF RF telemetry for end-to-end remote data monitoring. The developed mP/Gstat printed circuit board measures about 51/spl times/22/spl times/1mm thick, and is suitable for bench top or encapsulated use. Instrumentation developed in this paper will allow other researchers to develop implantable units. Evaluation in two- and three-electrode mPstat and two electrode mGstat modes with simulated biosensors showed overall average theoretical to actual recorded value differences of only 0.5859% reading (RDG) with a standard deviation (SD) of 0.5376% indicating excellent design performance. System testing in two-electrode mPstat mode with actual glucose sensors showed excellent stability and correlation to standard biosensor calibrations with an average difference of only 7.33% full scale (FS) with 3.65% SD, which is minimal considering two totally independent calibration instrument configurations.

Cones, spirals, and Möbius strips, in elliptically polarized light

The orientation of the ellipses in elliptically polarized light generally varies throughout space. In three dimensions, the orientation of an ellipse may be described by a 3-frame in which one frame axis is along the major axis of the ellipse, a second frame axis is along the minor axis of the ellipse, and the third frame axis is along the normal to the ellipse. These three axes are shown to generate cones, spirals, and Möbius strips, characterized by a total of 27 different topological indices. For ordinary ellipses (the vast majority) that are not on singular lines of circular or linear polarization 21 indices are non-zero. These indices, if independent, could collectively divide the field into 221=2,097,152 structurally different volumes separated by singular surfaces on which an index becomes undefined. We have, however, found selection rules that reduce the number of independent configurations to 140,608, and have demonstrated that large numbers of configurations appear in practice by harvesting more than 10,000 different configurations in a simulated random field. This structural proliferation is intrinsic to spatially varying elliptically polarized light, and is also found in non-random ellipse fields such as optical lattices. Other systems described locally by spatially varying 3-frames, such as liquid crystals or the dielectric constants of random media, may show a similar degree of structural proliferation.

Region-based shape analysis with tracked locations

This paper proposes a novel approach to shape analysis: using local reasoning about individual heap locations instead of global reasoning about entire heap abstractions. We present an inter-procedural shape analysis algorithm for languages with destructive updates. The key feature is a novel memory abstraction that differs from traditional abstractions in two ways. First, we build the shape abstraction and analysis on top of a pointer analysis. Second, we decompose the shape abstraction into a set of independent configurations, each of which characterizes one single heap location. Our approach: 1) leads to simpler algorithm specifications, because of local reasoning about the single location; 2) leads to efficient algorithms, because of the smaller granularity of the abstraction; and 3) makes it easier to develop context-sensitive, demand-driven, and incremental shape analyses.We also show that the analysis can be used to enable the static detection of memory errors in programs with explicit deallocation. We have built a prototype tool that detects memory leaks and accesses through dangling pointers in C programs. The experiments indicate that the analysis is sufficiently precise to detect errors with low false positive rates; and is sufficiently lightweight to scale to larger programs. For a set of three popular C programs, the tool has analyzed about 70K lines of code in less than 2 minutes and has produced 97 warnings, 38 of which were actual errors.

Reversed Monte Carlo simulation to XAFS spectra of liquid GeO2 polymorphs

X-ray absorption fine structure (XAFS) spectra of Ge-K edge of liquid GeO2 polymorphs (quartz-type and rutile-type GeO2) at temperature 1126 °C were collected at Beijing Synchrotron Radiation Facility (BSRF) and Spring-8. Reversed Monte Carlo (RMC) method was used to simulate the XAFS spectra with k2 weight. More than 200 independent configurations were used to do statistic analysis. There are 3.7 oxygen atoms surrounding Ge with Ge-O bond length of 1.74 Å for rutile-type GeO2 melt, and 3.6 oxygen atoms surrounding Ge with Ge-O bond length of 1.75 Å for quartz-type GeO2 melt. The long-range order was completely destroyed and partial short-range and intermediate-range order were kept in the two kinds of liquid GeO2 (at 1126 °C). Because partial Ge-O bonds were broken, the local atomic structures of liquid GeO2 were reconstructed. Besides the four-folded Ge-O coordination, three-folded and five-folded coordinations were also found. The liquid structures of the two GeO2 polymorphs are similar but not the same. The averaged Ge-O-Ge and O-Ge-O coordination triangles are larger in QGeO2 melt than in RGeO2 melt.

Impact of Reduced Vertical Separation Minimum Implementation on Airport System Infrastructures

This paper explores the impact of the reduced vertical separation minimum (RVSM) implementation on the airside of a large-airport system infrastructure. The hypothesis is that the frequencies of aircraft operations at the airport threshold, both for the departures and arrivals, will increase since almost double the number of aircraft can be operated in the same quantity of airspace when all flight levels in the RVSM environment are fully utilized. Simulation models are used to test these hypotheses. Airport systems with dependent parallel runways, intersecting runways, and independent parallel runway configurations are selected for our study. The analyses use the full-day traffic of the peak months at each airport as flight inputs and are conducted for both good and poor weather conditions. Not surprisingly, the results confirm that the RVSM implementation would aggravate delay on the airside of an airport system infrastructure, especially on the airfield where the capacity is limited and during the peak hours. The simulations provide insight into where improvements might be made. The methodology developed in this work to explore the factors that contribute to delay with the RVSM implementation is generally applicable to other airport systems.

A Particular Bit of Universality: Scaling Limits of Some Dependent Percolation Models

We study families of dependent site percolation models on the triangular lattice and hexagonal lattice ℍ that arise by applying certain cellular automata to independent percolation configurations. We analyze the scaling limit of such models and show that the distance between macroscopic portions of cluster boundaries of any two percolation models within one of our families goes to zero almost surely in the scaling limit. It follows that each of these cellular automaton generated dependent percolation models has the same scaling limit (in the sense of Aizenman-Burchard [3]) as independent site percolation on .

Topological susceptibility from the overlap

The chiral symmetry at finite lattice spacing of Ginsparg-Wilson fermionic actions constrains the renormalization of the lattice operators; in particular, the topological susceptibility does not require any renormalization, when using a fermionic estimator to define the topological charge. Therefore, the overlap formalism appears as an appealing candidate to study the continuum limit of the topological susceptibility while keeping the systematic errors under theoretical control. We present results for the SU(3) pure gauge theory using the index of the overlap Dirac operator to study the topology of the gauge configurations. The topological charge is obtained from the zero modes of the overlap and using a new algorithm for the spectral flow analysis. A detailed comparison with cooling techniques is presented. Particular care is taken in assessing the systematic errors. Relatively high statistics (500 to 1000 independent configurations) yield an extrapolated continuum limit with errors that are comparable with other methods. Our current value from the overlap is $\chi^{1/4} = 188 \pm 12 \pm 5 \MeV$.

PC-based automation systems: an example of application for the real-time control of blowing machines

Personal computers are nowadays being used even more frequently to implement time critical tasks in (distributed) automation systems. The achievement of such an interesting feature has been made possible thanks to several factors; the most important of them are: the impressive growth of the PC's performances, the availability of real-time operating systems and the introduction of programming languages suitable to develop control tasks. Moreover, the implementation of PC-based automation systems has been favoured by the recent considerable diffusion of field networks, which allow for the realization of manufacturer independent configurations. In this paper, after a general description of the PC-based automation systems, we consider, as an example of application, the automation of blowing machines producing PET bottles. The study, which has been carried out in the framework of a joint research project between the Italian National Council of Research and an Italian firm, was aimed at investigating the possibility of using a PC-based automation system for the blowing machines. The adoption of such a type of system has been carefully evaluated by means of a preliminary theoretical analysis. Then, a test machine has been implemented to collect data and performance figures by carrying out experiments and measures. Finally, the results obtained have been compared with those of an equivalent machine equipped with a traditional automation system based on a Programmable Logic Controller.

On the meaning of Parisi's functional order parameter

The author has recently proved that a famous formula discovered by G. Parisi gives at any temperature the correct value for the limiting free energy of a large class of mean field models for spin glasses (a class which contains in particular the Sherrington–Kirkpatrick model). Here we prove rigorously that (generically) the “functional order parameter” occuring in this formula can be interpreted as predicted by Parisi, namely as representing the limiting distribution of the overlap of two independent configurations. To cite this article: M. Talagrand, C. R. Acad. Sci. Paris, Ser. I 337 (2003).

The CO-CO dimer as a nonrigid molecular system

A model for describing the spectrum of internal motions of the carbon monoxide dimer CO-CO is constructed with allowance for nonrigid transitions between its independent equilibrium configurations. The model construction is based on use of the symmetry-group chain methods currently being developed, which allow one to describe internal dynamics without resorting to equations of motion.

Long period gratings in multimode optical fibers: application in chemical sensing

We propose and demonstrate a new technique for evanescent wave chemical sensing by writing long period gratings in a bare multimode plastic clad silica fiber. The sensing length of the present sensor is only 10 mm, but is as sensitive as a conventional unclad evanescent wave sensor having about 100 mm sensing length. The minimum measurable concentration of the sensor reported here is 10 nmol/l and the operating range is more than 4 orders of magnitude. Moreover, the detection is carried out in two independent detection configurations viz., bright field detection scheme that detects the core-mode power and dark field detection scheme that detects the cladding mode power. The use of such a double detection scheme definitely enhances the reliability and accuracy of the results. Furthermore, the cladding of the present fiber need not be removed as done in conventional evanescent wave fiber sensors.

Structures and melting of Cu n ( n=13, 14, 19, 55, 56) clusters

Constant-energy molecular dynamics simulation was utilized to study the most stable geometrical structures, binding energy, melting, phase changes of Cu n ( n=13, 14, 19, 55, 56) clusters. These sizes form shell and near shell structures. The cohesion of clusters is modeled by an embedded-atom potential, which contains many-body atomic interaction terms. Phase space coordinates which are generated along high-energy trajectories are used as the initial configurations (500 independent configurations) for thermal quenching in order to obtain the most stable isomers. The melting temperatures of the clusters are estimated. The melting-like transition is described in terms of relative root-mean-square bond-length fluctuations, specific heats, and caloric curves.

The potential energy landscape in the Lennard-Jones binary mixturemodel

The potential energy landscape in the Kob–Andersen Lennard-Jones binary mixturemodel has been studied carefully from the liquid down to the supercooled regime, fromT = 2down to 0.46. One thousand independent configurations along the time evolutionlocus have been examined at each temperature investigated. From the startingconfiguration, we searched for the nearest saddle (or quasi-saddle) andminimum of the potential energy. The vibrational densities of states for thestarting and the two derived configurations have been evaluated. Besides thenumber of negative eigenvalues of the saddles other quantities show somesignature of the approach of the dynamical arrest temperature.

Damage analysis in composite laminates by using an interface element

A numerical analysis is performed to study an impact damage accumulation problem in composite laminates. A special three-dimensional interface element is developed based on the cohesive model. The element enables the mesh independent configurations of a crack to be calculated and can treat contact problems of the delaminated area. The energy stored in the element per unit area is defined as a function of continuous relative displacements of the delaminated surface. The energy stored before the perfect separation of the interface is equal to the interlaminar fracture toughness. The softening cohesive relations between the tractions and the relative displacements are given by differentiating the energy function with respect to the relative displacements. The maximum value of traction may coincide with interfacial bonding strength. The element is incorporated in a commercially available finite element code. Crack propagation problems under pure Mode I and Mode II loading conditions for three-dimensional models are calculated to show the validity of the present element. The convergence of solution with mesh refinement is examined. The analytical solutions converge smoothly and agree well with the theoretical ones. The present method may be a good tool to simulate the damage accumulation problem of CFRP laminates.

Energetics and structures of small clusters: Pt N, N=2–21

The Voter and Chen version of an embedded-atom model, derived by fitting to experimental data of both diatomic molecule and bulk platinum simultaneously, has been applied to study the locally stable structures, energies and growth patterns of small platinum clusters in the size range of N=2–21. Using molecular dynamics and thermal quenching simulations, the global minima and the other locally stable structures have been distinguished from those stationary structures that correspond to saddle points of the potential energy surface. Ten thousand independent initial configurations generated at high temperatures (about 2600 K) were used to obtain the number of isomers and the probabilities of sampling different basins of attractions, for each size of the clusters. Their energy spectra have been analyzed. Comparisons have been made with the results of previous calculations using electronic structure and empirical potential methods. Although many of the lowest energy structures correspond to icosahedral growth, a number of new structures have been identified for N=15, 16, 17, 18, 20 and 21. It has been found that the lowest energy structures are not always the most probable isomers for each size.

On the partition function of a directed polymer in a Gaussian random environment

The purpose of this work is the study of the partition function of a -dimensional lattice directed polymer in a Gaussian random environment being the inverse of temperature). In the low-dimensional cases , we prove that for all , the renormalized partition function converges to 0 and the correlation of two independent configurations does not converge to 0. In the high dimensional case ( ), a lower tail of has been obtained for small . Furthermore, we express some thermodynamic quantities in terms of the path measure alone.

Rapid Calculation of the Structures of Solutions with ab Initio Interaction Potentials

A non-Boltzmann sampling method is demonstrated for efficient calculation of structural properties of models having ab initio interaction potentials. A sample of independent configurations is generated with a molecular dynamics simulation using an approximate potential. Ab initio calculations, done only at the sampled configurations, are used to correct the thermal averages of any mechanical variable for the difference between the approximate and ab initio potentials. Tests of the method show that relatively few ab initio calculations are needed to obtain significant structural information and that bootstrap estimates of the uncertainties are reasonable. As a further test, we use results from a previous calculation of the hydration free energy of Na + at 973 K and 0.535 g/cm 3 to calculate the Na-O pair correlation function and coordination number for an ab initio model of Na + -H 2 O interactions.

Structures and energetics Of Pd n ( n=2–20) clusters using an embedded-atom model potential

We have studied the structure and energetics of the stable isomers of Pd n ( n=2–20) clusters by using molecular dynamics and slow-quenching techniques. Cohesion of the clusters is modeled by an embedded-atom potential due to Voter and Chen, which contains many-body atomic interactions. The isomers' statistics are obtained from 10 000 independent initial configurations, which have been generated along a high-energy trajectory (the chosen energy value is high enough to melt the cluster). The internal kinetic energy of these initial conditions is removed slowly. Because of this slow minimization process the locally stable isomers are separated from those meta-stable ones. Probabilities belonging to sampling the basins of attractions of each isomer are computed, and compared with each other. Furthermore, the spectrum, which is formed by isomers' energies, is analyzed.