M. J. de Oliveira

SAMPLING CONFIGURATIONS IN MONTE CARLO SIMULATIONS FOR QUANTUM MECHANICAL STUDIES OF SOLVENT EFFECTS

Discrete models obtained from computer simulation are in increase use to study solvent effects. The approach consists of generating supermolecular structures for quantum mechanical calculations. The properties of the solute are calculated as an ensemble average over configurations generated by the simulation. An analysis of the efficiency of the simulation shows that the number of configurations necessary for the ensemble average can be reduced drastically. As an application to solvatochromism, the 1 U. calculated spectral shift of the B p y p transition of benzene in water is shown to be 2 u

Nonequilibrium spin models with Ising universal behaviour

The properties of a family of nonequilibrium spin models with up-down symmetry on a square lattice are determined by a mean-field pair approximation and by Monte Carlo simulation. The phase diagram in the parameter space displays a critical line that terminates at a first-order critical point. It is found that the critical exponents are the same as those of the equilibrium Ising model.

Inferring statistical complexity in the dripping faucet experiment

We have used topological e-machine reconstruction to calculate statistical complexity associated to period five orbits in the dripping faucet experiment. Basic structures related to the periodic movement and to peripheral tracks in the corresponding directed graphs were identified.

Glassy states in lattice models with many coexisting crystalline phases

We study the emergence of glassy states after a sudden cooling in lattice models with short-range interactions and without any a priori quenched disorder. The glassy state emerges whenever the equilibrium model possesses a sufficient number of coexisting crystalline phases at low temperatures, provided the thermodynamic limit be taken before the infinite time limit. This result is obtained through simulations of the time relaxation of the standard Potts model and some exclusion models equipped with a local stochastic dynamics on a square lattice.

Diffusion-limited annihilation and the reunion of bounded walkers

We study the long time behavior of a one-species reaction-diffusion process kA ®lA where k particles coalesce into l particles. The asymptotic time behavior of the density of particles is derived by mapping the reaction-diffusion process into the problem of the reunion of k random walkers bounded to move in a limited region

An extinction-survival-type phase transition in the probabilistic cellular automaton p182?q200

We investigate the critical behaviour of a probabilistic mixture of cellular automata (CA) rules 182 and 200 (in Wolframs enumeration scheme) by mean-field analysis and Monte Carlo simulations. We found that as we switch off one CA and switch on the other by the variation of the single parameter of the model, the probabilistic CA (PCA) goes through an extinction?survival-type phase transition, and the numerical data indicate that it belongs to the directed percolation universality class of critical behaviour. The PCA displays a characteristic stationary density profile and a slow, diffusive dynamics close to the pure CA 200 point that we discuss briefly. Remarks on an interesting related stochastic lattice gas are addressed in the conclusions.

Field behaviour of the XY chiral model on a Cayley tree

The authors present a study of the XY chiral model on a Cayley tree in the presence of an external uniform magnetic field. They calculate the phase diagram at zero temperature from a corresponding 1D mapping. Ferromagnetic commensurate and incommensurate modulated phases as well as chaotic structures are present in the zero temperature limit. In this case they also obtain a devils staircase and determine the route to chaos, which is in agreement with Feigenbaums scenario. For finite temperatures, the phase diagrams are obtained from a 2D mapping. The chaotic behaviour is present only at low temperatures.

Ising spin glass in the Bethe approximation at zero temperature

We study the properties of the Ising spin glass in the Bethe approximation at zero temperature for the case of a ±J distribution of bonds. We show that there is an infinite number of solutions for the probability distribution function of the effective field. Each one is a sum of 2N+1 delta functions located at Jn/N, n=0,±1, ±2,…,±N. In the limit N→∞ we obtain a solution with a continous part in addition to delta functions located at 0 and ±J. For each case we calculate the spin glass order parameter, the energy and entropy.

Fluctuation-dissipation theorem and the linear Glauber model.

We obtain exact expressions for the two-time autocorrelation and response functions of the -dimensional linear Glauber model. Although this linear model does not obey detailed balance in dimensions d > or = 2, we show that the usual form of the fluctuation-dissipation ratio still holds in the stationary regime. In the transient regime, we show the occurrence of aging, with a special limit of the fluctuation-dissipation ratio, x(infinity) = 1/2, for a quench at the critical point.

Renormalization group of probabilistic cellular automata with one absorbing state

We apply a recently proposed dynamically driven renormalization-group scheme to probabilistic cellular automata having one absorbing state. We find just one unstable fixed point with one relevant direction. In the limit of small transition probability one of the cellular automata reduces to the contact process, revealing that the cellular automata are in the same universality class as that process, as expected. Better numerical results are obtained as the approximations for the stationary distribution are improved.