Coarse-grained Techniques Research Articles

Effective field theory of the zero-temperature triangular-lattice antiferromagnet: A monte carlo study

Using a Monte Carlo coarse-graining technique introduced by Binder [Z. Phys. B 43, 119 (1981)], we have explicitly constructed the continuum field theory for the zero-temperature triangular Ising antiferromagnet. We verify the conjecture that this is a Gaussian theory of the height variable in the interface representation of the spin model. We also measure the height-height correlation function and deduce the stiffness constant. In addition, we investigate the nature of defect-defect interactions at finite temperatures, and find that the two-dimensional Coulomb gas scenario applies at low temperatures.

Approaches to optical Internet packet switching

Wavelength-division multiplexing is currently being deployed in telecommunications networks in order to satisfy the increased demand for capacity brought about by the explosion in Internet use. The most widely accepted network evolution prediction is via an extension of these initial predominantly point-to-point deployments, with limited system functionalities, into highly interconnected networks supporting circuit-switched paths. While current applications of WDM focus on relatively static usage of individual wavelength channels, optical switching technologies enable fast dynamic allocation of WDM channels. The challenge involves combining the advantages of these relatively coarse-grained WDM techniques with emerging optical switching capabilities to yield a high-throughput optical platform directly underpinning next-generation networks. One alternative longer-term strategy for network evolution employs optical packet switching, providing greater flexibility, functionality, and granularity. This article reviews progress on the definition of optical packet switching and routing networks capable of providing end-to-end optical paths and/or connectionless transport. To date the approaches proposed predominantly use fixed-duration optical packets with lower-bit-rate headers to facilitate processing at the network-node interfaces. Thus, the major advances toward the goal of developing an extensive optical packet-switched layer employing fixed-length packets are summarized, but initial concepts on the support of variable-length IP-like optical packets are also introduced. Particular strategies implementing the crucial optical buffering function at the switching nodes are described, motivated by the network functionalities required within the optical packet layer.

Coarse-graining analysis of the Berreman anchoring.

By means of a coarse-graining technique, we derive the effective anchoring energy of a nematic liquid crystal in contact with a macroscopically corrugated surface imposing a weak degenerate planar anchoring. For coarse-grained nematic director's profiles that vary only in the direction perpendicular to the average surface's plane, our results generalize those already known by including the anisotropy of the elastic constants. In the general case, on the contrary, we show that extra surface gradient terms appear that generally are not negligible.

Locality analysis for parallel C programs

Many parallel architectures support a memory model where some memory accesses are local and, thus, inexpensive, while other memory accesses are remote and potentially quite expensive. In the case of memory references via pointers, it is often difficult to determine if the memory reference is guaranteed to be local and, thus, can be handled via an inexpensive memory operation. Determining which memory accesses are local can be done by the programmer, the compiler, or a combination of both. The overall goal is to minimize the work required by the programmer and have the compiler automate the process as much as possible. This paper reports on compiler techniques for determining when indirect memory references are local. The locality analysis has been implemented for a parallel dialect of C called EARTH-C, and it uses an algorithm inspired by type inference algorithms for fast points-to analysis. The algorithm statically estimates when an indirect reference via a pointer can be safely assumed to be a local access. The locality inference algorithm is also used to guide the automatic specialization of functions in order to take advantage of locality specific to particular calling contexts. In addition to these purely static techniques, we also suggest fine-grain and coarse-grain dynamic techniques. In this case, dynamic locality checks are inserted into the program and specialized code for the local case is inserted. In the fine-grain case, the checks are put around single memory references, while in the coarse-grain case the checks are put around larger program segments. The static locality analysis and automatic specialization has been implemented in the EARTH-C compiler, which produces low-level threaded code for the EARTH multithreaded architecture. Experimental results are presented for a set of benchmarks that operate on irregular, dynamically allocated data structures. Overall, the techniques give moderate to significant speedups, with the combination of static and dynamic techniques giving the best performance overall.

Microhydrodynamics with Dissipative Particle Dynamics

ABSTRACTDissipative particle dynamics is essentially a coarse-grained molecular dynamic simulation technique that captures the essential physics with considerably less computer effort. We have given a sound theoretical foundation to the technique with respect to the equilibrium and hydrodynamic properties. In this paper we further explore the connection of the model parameters of DPD with the underlying microscopic dynamics for the case of a simple model of a solid. This provides some insight into the difficulties of interpretation of DPD simulations.

Coordination in Coarse-Grained Decomposition

The dominance of parallel computers in the supercomputer field provides great incentive for the development of parallel algorithms in mathematical programming. Fortunately, parallel computers provide a good medium for the implementation of many types of decomposition algorithms for large-scale optimization. In this paper, a variety of coordination methods are studied for coarse-grained decomposition techniques. Some are variants of existing approaches that are designed to enhance rates of convergence and/or parallel efficiency, while others are novel methods that try to take maximum advantage of the parallel environment.

Real-time path-integral simulation of vibrational transition probabilities of small molecules in clusters: Theory and application to Br2 in Ar

A discretized real-time Feynman path integral is applied to a system containing a diatomic in a small cluster of solvent molecules. The system considered consists of Br2 in Ar. An adiabatic separation of variables is assumed. The solvent as well as the Br2 center of mass are gathered into a vector called bath or solvent coordinates. The forced oscillator approximation is used to analytically obtain the vibrational contribution to the transition amplitude. The discretized real-time propagators (bath dependent only, since the vibrational part is carried out analytically) are highly oscillatory and, therefore, not suitable for Monte Carlo calculations. The coarse-graining technique introduced by Filinov and developed by Freeman and Doll, and Miller is employed to make the integrals more suitable for Monte Carlo calculations. The computations are carried out for five different times. For each time, we study the convergence of the technique for a range of Gaussian widths used as conditioning functions. We also examine the convergence as a function of the number of points in the discretized description of the solvent path.

Extended parallelism in the Gr�bner basis algorithm

This paper presents a new parallel implementation to compute Grobner bases utilizing two different forms of parallelism. A coarse-grain technique developed by Jean-Phillipe Vidal expands and reducesS-polynomials in parallel. A finegrain technique, proposed by Melenk and Neun, constructs a pipeline of processors to overlap execution of the reduction operations. A hybrid algorithm that outperforms both of the original approaches is presented. The combined algorithm requires the user to select the appropriate allocation of processors to the two styles of parallelism, and uses this static assignment throughout the computation. The paper also discusses the design and implementation approaches used to construct an efficient version of this algorithm.

A simple coarse-graining technique for the Monte Carlo evaluation of highly oscillatory integrals

A simple technique for conditioning highly oscillatory integrands to make them suitable for Monte Carlo evaluation is presented and investigated numerically for one-dimensional problems. The technique involves averaging the integrand uniformly over a local region if the integrand is oscillating rapidly. In regions of slow oscillation the unconditioned integrand is employed. The averaging region is chosen to be one oscillation. We argue that the proper choice of the averaging region is important in determining the suitability of the conditioned integrand for use in Monte Carlo calculations.

Thermoluminescence dating of coarse-grain quartz from the Malan loess at Zhaitang section, China

Thermoluminescence (TL) ages were obtained for five loess samples taken from the Zhaitang section near Beijing, China, using the coarse-grain quartz technique. The paleodose values have been determined by the method of total sample bleaching and regeneration of the TL growth curve. The method appears to be suitable for the age determination of loess samples up to about 150,000 yr where the annual dose-rate values are of the order 3–4 mGy/yr. This limit is a function of the total accumulated dose. The ages are in good agreement with those obtained by a fine-grain TL technique and are consistent with geological and geomagnetostratigraphic evidence.

Hamiltonian of a slightly relativistic plasma

In the past, treatments of plasma to order v2/c2, where v is the thermal speed of the particles, have relied on a Lagrangian formalism or on coarse graining techniques. It is shown that a Hamiltonian formulation where only particle degrees of freedom are considered is possible and a Hamiltonian to order v2/c2 is obtained. This Hamiltonian is valid within the limits of the random phase approximation.

Stochastic theory for molecular collisions: Application to the CO–He system

Vibration–rotation inelasticity in the CO–He collision system is studied within the stochastic formulation. Cross sections are obtained for purely rotational transitions using a modified electron gas potential. Vibration–rotation cross sections have been calculated in the energy range 2200 cm−1<E<4000 cm−1. At the higher energy, a total of 76 molecular states are energetically accessible. A comparison with earlier results is made, and coarse graining techniques for the treatment of large problems are utilized.