International Journal of Modern Physics C

We study a family of deterministic models for highway traffic flow which generalize cellular automaton rule 184. This family is parameterized by the speed limit m and another parameter k that represents a "degree of aggressiveness" in driving, strictly related to the distance between two consecutive cars. We compare two driving strategies with identical maximum throughput: "conservative" driving with high speed limit and "aggressive" driving with low speed limit. Those two strategies are evaluated in terms of accident probability. We also discuss fundamental diagrams of generalized traffic rules and examine limitations of maximum achievable throughput. Possible modifications of the model are considered.

Recent empirical and theoretical studies have shown that simple parameters characterizing constraint satisfaction problems predict whether they have a solution and the cost to solve them, on average. This paper examines the effectiveness of using these predictions as a heuristic for solving the graph coloring problem. Specifically, by adding some global information on the consequences of various choices, the use of these parameters can reduce the search required to find a solution. Current limitations of this approach, due to the high variance associated with the predictions, are also presented. More generally, observations of universal behaviors analogous to physical phase transitions can be applied to improve search methods.

We study numerically the nature of the retrieval attractors in an asymmetrically diluted Hopfield neural network through the damage propagation technique. We consider the damage evolution of two replicas, initially very close to each other and having both finite projection with one memorized pattern. By analyzing the asymptotic behavior of the damage, we characterize the dynamical nature of the retrieval attractors. We found, in the recognition phase, a dynamical phase transition separating chaotic and fixed point retrieval trajectories. We also present a conjugate field h associated with the damage, which destroys the dynamical transition and whose corresponding susceptibility presents a sharp peak at the critical parameter.

Two basic approaches to the cluster counting task in the percolation and related models are discussed. The Hoshen–Kopelman multiple labeling technique for cluster statistics is redescribed. Modifications for random and aperiodic lattices are sketched as well as some parallelized versions of the algorithm are mentioned. The graph-theoretical basis for the spanning tree approaches is given by describing the breadth first search and depth first search procedures. Examples are given for extracting the elastic and geometric "backbone" of a percolation cluster. An implementation of the "pebble game" algorithm using a depth first search method is also described.

The formation of point defects in diamond induced by an energetic displacement of a carbon atom out of its lattice site and the relaxation of the thereby disrupted crystal are studied by molecular dynamics simulations with the Tersoff potential. The displacement energy for Frenkel pair creation is calculated to be 52 eV, in agreement with experiments. It is found that the <100> split interstitial, with a bonding configuration which resembles graphite, is the most stable defect, and the disrupted region around it is rich in sp²-like (graphitic) bonds. This region extends several nanometers and is likely to be the elementary electrically conductive cell experimentally found in radiation-damaged diamond.

This study investigates the possibility of using an Artificial Neural Network (ANN) for reconstructing Positron Emission Tomography (PET) images. The network is trained with simulated data which include physical effects such as attenuation and scattering. Once the training ends, the weights of the network are held constant. The network is able to reconstruct every type of source distribution contained inside the area mapped during the learning. The reconstruction of a simulated brain phantom in a noiseless case shows an improvement if compared with Filtered Back-Projection reconstruction (FBP). In noisy cases there is still an improvement, even if we do not compensate for noise fluctuations. These results show that it is possible to reconstruct PET images using ANNs. Initially we used a Dec Alpha; then, due to the high data parallelism of this reconstruction problem, we ported the learning on a Quadrics (SIMD) machine, suited for the realization of a small medical dedicated system. These results encourage us to continue in further studies that will make possible reconstruction of images of bigger dimension than those used in the present work (32 × 32 pixels).

Simulation of Elastic Light Scattering from arbitrary shaped particles in the resonance region (i.e., with a dimension of several wavelengths of the incident light) is a long standing challenge. By employing the combination of a simulation kernel with low computational complexity, implemented on powerful High Performance Computing systems, we are now able to push the limits of simulation of scattering of visible light towards particles with dimensions up to 10 micrometers. This allows for the first time the simulation of realistic and highly relevant light scattering experiments, such as scattering from human red — or white blood cells, or scattering from large soot — or dust particles. We use the Discrete Dipole Approximation to simulate the light scattering process. In this paper we report on a parallel Fast Discrete Dipole Approximation, and we will show the performance of the resulting code, running under PVM on a 32-node Parsytec CC. Furthermore, as an example we present results of a simulation of scattering from human white blood cells. In a first approximation the Lymphocyte is modeled as a sphere with a spherical inclusion. We investigate the influence of the position of the inner sphere, modeling the nucleus of a Lymphocyte, on the light scattering signals.

We present new procedures in REDUCE language using the EXCALC package (adapted for IBM-PC machines) for algebraic programming in the hamiltonian formulation of Einstein–Maxwell equations. The dynamic and the constraint equations containing specific terms of the interaction of gravity with source-free electromagnetic field are translated into computer procedures. The results obtained by processing some examples of space-time models are presented.

Iterative transportation microsimulations adjust traveler route plans by iterating between a microsimulation and a route planner. At each iteration, the route planner adjusts individuals' route choices based on the preceding microsimulations. Empirically, this process yields good results, but it is usually unclear when to stop the iterative process when modeling real-world traffic. This paper investigates several criteria to judge relaxation of the iterative process, emphasizing criteria related to traveler decision-making.

Optimization strategies based on simulated annealing and its variants have been extensively applied to the traveling salesman problem (TSP). Recently, there has appeared a new physics-based metaheuristic, called the microcanonical optimization algorithm (μO), which does not resort to annealing, and which has proven a superior alternative to the annealing procedures in various applications. Here we present the first performance evaluation of μO as applied to the TSP. When compared to three annealing strategies (simulated annealing, microcanonical annealing and Tsallis annealing), and to a tabu search algorithm, the microcanonical optimization has yielded the best overall results for several instances of the euclidean TSP. This confirms μO as a competitive approach for the solution of general combinatorial optimization problems.

The percolation phenomena on two-dimensional square lattice is considered. The quotient χ of connectivity length ξ^> above percolation threshold p_c and ξ^< below p_c at the same small distance Δp is discussed. The results of two different algorithms and programs and agreement with theoretical/mathematical predications is presented, in contrast to previous contradictory Monte Carlo simulation results.

Molecular self-assembly is frequently encountered in biochemical systems generating higher order structures with well-defined functionalities. However, the driving forces underlying these processes are not well understood.

The Lattice Molecular Automaton (LMA) is a deterministic computational tool suitable for simulation of self-organization processes in large scale, molecular systems. This paper introduces the basic computational concepts needed to formulate molecular dynamics and self-assembly in a discrete field, cellular automaton environment: Molecular objects are encoded as data structures on a 2D triangular lattice. Propagating force particles together with kinetic and potential energy terms define simulation objects that specify molecular dynamics and force field properties. As an example, the simulation of polymer dynamics in an aqueous environment is shown.

In this paper we focus on the mathematical and algorithmic formulation of a variety of intra- and intermolecular interactions. Thermodynamical characteristics together with a variety of other physico-chemical properties of the LMA are discussed in detail in Ref. 1.

This work analyzes the stability and accuracy of multistep methods, either for serial or parallel calculations, applied to molecular dynamics simulations. Numerical testing is made by evaluating the equilibrium configurations of mono-elemental crystalline lattices of metallic and semiconducting type (Ag and Si, respectively) and of a cubic CuY compound.

We have analyzed low and high temperature series expansions for the three-dimensional Ising model on the simple cubic lattice. Our analysis of Butera and Comi's new 21-term high temperature series yields and from the 32-term low temperature series of Vohwinkel we find K_c=0.22167±0.00002, consistent with the high temperature series but with larger error bars. We discuss the reasons for the larger error bars on the low temperature side and compare these values with estimates from other series analyses and from simulations.