International Journal of Modern Physics C

Q2R cellular automata are combined with Kawasaki dynamics to give an Ising ferromagnet with conserved magnetization and conserved energy. In its lattice gas interpretation, it may serve to describe the diffusion of hydrogen ions on the surface of globular proteins. Our computer simulations test to what extent the phase transition at the Curie point is still visible in small 10×10 lattices and t~10⁴.

Spin–spin correlation functions <S(0)S(R)> as dependent on interatomic distance R are studied in the random-site two-dimensional Ising S=1/2 ±J system. Oscillations of the correlation functions are found, which is not a case in the random-bond system.

A Hopfield-type neural network has content addressable memory which emerges from its collective properties. I reinvestigate the controversial question of its critical storage capacity at zero temperature. To locate the discontinuous transition from good retrieval to bad retrieval in infinite systems the decreasing average quality of retrieved information is traced until it falls below a threshold. The cutoff points found for different system sizes are extrapolated towards infinity and yield α_c=0.143±0.002.

A two species predator–prey model based on the Lotka–Volterra equations is proposed, where the fitness of an individual animal depends upon the relative strength of its genes. Simulations of the model show that the system passes from the standard oscillatory solution of the Lotka–Volterra equations into a steady-state regime, which exhibits many of the characteristics of self-organized criticality, including a 1/f power spectrum.

An accurate multiaffine analysis of 23 foreign currency exchange rates has been performed. The roughness exponent H₁ which characterizes the excursion of the exchange rate has been numerically measured. The degree of intermittency C₁ has been also estimated. In the (H₁,C₁) phase diagram, the currency exchange rates are dispersed in a wide region around the Brownian motion value (H₁=0.5,C₁=0) and have a significantly intermittent component (C₁≠0).

This work presents a simple simulation of the differences in male and female mortality and ageing. The simulation is based on Penna's bit-string model of biological ageing.

We investigate local update algorithms for the fully frustrated XY model on a square lattice. In addition to the standard updating procedures like the Metropolis or heat bath algorithm we include overrelaxation sweeps, implemented through single spin updates that preserve the energy of the configuration. The dynamical critical exponent (of order two) stays more or less unchanged. However, the integrated autocorrelation times of the algorithm can be significantly reduced.

A lattice model is used to simulate the dependence of the condensation temperature of ternary mixtures (water-oil-surfactants) on varying tail length. Nonionic single-tailed amphiphiles are assumed. We have obtained the result that the longer the tail length, the higher is the condensation temperature. We also discuss the influence of the head–head interaction on that temperature.

In numerical large-scale hydrodynamics calculations, such as the description of a supernova explosion, instantaneous thermalization of the fluid matter is assumed independently of the size of the volume element used in the calculation. One expects, however, the appearance of transient processes such as convection currents, vortices, and other collective motion on smaller and smaller scales, which can delay equilibration. To account for these effects in a simple one-dimensional hydrodynamical calculation, we introduce retardation in the hydrodynamic equations and show that, when strong shocks are present, such effects may have considerable influence on the evolution of the system.

Numerical simulations of suspensions often suffer from the fact that the simulated systems are rather small compared to experimental setups. We present a numerical scheme for non-Brownian particle-liquid mixtures in three dimensions at particle Reynolds numbers between 0.01 and 20 and describe its parallel implementation. The fluid equations are solved by a time-explicit pressure-implicit Navier–Stokes algorithm and the particle motion is tracked by molecular-dynamics methods. The two are coupled by imposing no-slip boundary conditions between the particles and the fluid. We integrate the stress distribution on the particle surface numerically to obtain forces and torques. The building blocks of the algorithm are local and scalable and we have reached particle numbers up to 10⁶ (1.41*10⁸ fluid nodes) on a 512 node CRAY-T3E. We compare our simulation results to theoretical predictions and experimental data and find good agreement for particle volume fractions up to 0.30.

We present a model of a three-step food chain. Population A grows at a certain rate and at another rate is eaten by the population B which has its own birth rate but is, in turn, eaten at a different rate by the population C. The dynamics of the model is given by a set of differential equations and via Monte Carlo simulations. Our system undergoes sudden cataclysms in the form of partial destruction of one of the populations. We show that there exist threshold values for the possible percentage of destroyed populations, above which the system returns to its previous state, thus showing a self-regulatory character.