Noé Almarza | CSIC (Consejo Superior de Investigaciones Científicas-Spanish National Research Council) (original) (raw)

Papers by Noé Almarza

The Journal of Chemical Physics, 2009

Using Monte Carlo Simulation and fundamental measure theory we study the phase diagram of a two-d... more Using Monte Carlo Simulation and fundamental measure theory we study the phase diagram of a two-dimensional lattice gas model with a nearest neighbor hard core exclusion and a next-to-nearest neighbors finite repulsive interaction. The model presents two competing ranges of interaction and, in common with many experimental systems, exhibits a low density solid phase, which melts back to the fluid phase upon compression. The theoretical approach is found to provide a qualitatively correct picture of the phase diagram of our model system.

The Journal of Chemical Physics, 2013

A generic lattice model for systems containing particles interacting with short-range attraction ... more A generic lattice model for systems containing particles interacting with short-range attraction long-range repulsion (SALR) potential that can be solved exactly in one dimension is introduced.

The Journal of Chemical Physics, 2007

We propose an efficient algorithm to perform Monte Carlo simulations of dense systems using multi... more We propose an efficient algorithm to perform Monte Carlo simulations of dense systems using multiple particle moves. The method is intended to be used in the atomistic simulation of complex systems, where the computing requirements for a single simulation run make advisable the use of parallel computing. The algorithm is based on the use of steps in which all the particle positions of the system are perturbed simultaneously. A division of the system in clusters of particles is performed, using a bonding criterion which makes feasible that the acceptance or rejection of the new particle coordinates can be carried out independently for each cluster.

The European Physical Journal B - Condensed Matter, 2003

The structure, thermodynamics and the ferromagnetic phase transition of a positionally frozen dis... more The structure, thermodynamics and the ferromagnetic phase transition of a positionally frozen disordered Heisenberg spin system are studied by means of extensive Monte Carlo calculations in combination with finite size scaling techniques, as well as resorting to the Replica Ornstein-Zernike formalism. The system is formed by a collection of Heisenberg spins whose spatial distribution corresponds to a soft sphere fluid with its particle positions frozen at a certain quench temperature. The spin orientations are allowed to equilibrate at a given equilibrium temperature. If the quench and equilibrium temperatures are similar the properties of the positionally frozen system are practically indistinguishable from those of the fully equilibrated Heisenberg spin fluid. On the other hand, one observes that as the quenching temperature of the spatial degrees of freedom increases, so does the Curie temperature of the Heisenberg spins. The theory fails to reproduce the location of the ferromagnetic transition, despite its relative accuracy in the determination of the orientational structure in the supercritical region.

Physical Review E, 1999

The phase diagram of mixtures of hard spheres with additive diameters is studied. The case of ver... more The phase diagram of mixtures of hard spheres with additive diameters is studied. The case of very different sizes is treated by means of mapping the two component system on a one component problem. In this monocomponent system large particles are explicitly considered, whereas the effects of the small component are included through an additional effective interaction potential between large particles. The effective potential is used to analyze the phase diagram of the mixture by means of computer simulation techniques. Results for the behavior at low density of small spheres seem to indicate that no fluid-fluid equlibria occur. On the other hand, the results show how this kind of mixture can exhibit equilibria between isostructural crystalline phases.

Physical Review E, 2001

Phase transitions of lattice models of ionic crystals are studied by computer simulation. The nat... more Phase transitions of lattice models of ionic crystals are studied by computer simulation. The nature of order-disorder transitions on different crystal structures is established and compared with the behavior of related Ising models. It is found that for both, continuous and first order transitions the basic features seem to be similar to those of Ising systems.

The Journal of chemical physics, 2009

We propose an efficient algorithm to sample the volume in Monte Carlo simulations in the isobaric... more We propose an efficient algorithm to sample the volume in Monte Carlo simulations in the isobaric-isothermal ensemble. The method is designed to be applied in the simulation of hardcore models at high density. The algorithm is based in the generation of clusters of particles. At the volume change step, the distances between pairs of particles belonging to the same cluster do not change. This is done by rescaling the positions of the center of mass of each cluster instead of the position of each individual particle. We have tested the performance of the algorithm by simulating fluid and solid phases of hard spheres, finding that in both cases the algorithm is much more efficient than the standard procedure. Moreover, the efficiency of the method measured in terms of correlation "time" does not depend on the system size in contrast with the standard method, in which the sampling becomes rapidly inefficient as the system size increases. We have used the procedure to compute with high precision the equation of state of the face-centered cubic phase of the hard sphere system for different system sizes. Using these results we have estimated the equation of state at the thermodynamic limit. The results are compared with different equations of state proposed in the literature.

Physical Review Letters, 2002

ABSTRACT

Molecular Physics, 1998

An equation of state (EOS) for complex hard body fluids is developed by using as input the virial... more An equation of state (EOS) for complex hard body fluids is developed by using as input the virial coefficients: this has been called the CY5 EOS. After comparing with molecular simulation data of complex hard body systems, it is concluded that this is a very accurate route for predicting thermodynamic behaviour of such fluid phases. It corrects well established EOSs, like the Carnaham-Starling type equations for had spheres and non-spherical bodies. In addition, it gives new insight into controversial problems, like the demixing phenomena of hard sphere mixtures, produced by large size ratios or the presence of non-additive interactions.

The criticality of self-assembled rigid rods on triangular lattices is investigated using Monte C... more The criticality of self-assembled rigid rods on triangular lattices is investigated using Monte Carlo simulation. We find a continuous transition between an ordered phase, where the rods are oriented along one of the three (equivalent) lattice directions, and a disordered one. We conclude that equilibrium polydispersity of the rod lengths does not affect the critical behavior, as we found that the criticality is the same as that of monodisperse rods on the same lattice, in contrast with the results of recently published work on similar models.

The Journal of chemical physics, 2007

The Journal of chemical physics, 2005

Molecular Physics, 1992

Standard simulation techniques often show nonergodic trajectories when sampling systems with high... more Standard simulation techniques often show nonergodic trajectories when sampling systems with high energy barriers. The slow convergence of the configurational thermal averages leads to extended simulations at the limit of computational resources, and uncertainty is introduced in the final results. In this paper we devise a strategy for avoiding these problems, based on performing a set of replica simulations of the same system. The initial configurations are obtained according to the ensemble equilibrium distribution function. We propose to carry out an annealing process from a reference ergodic system to the actual system. At each step of the annealing trail we sample each replica with a standard simulation algorithm, and by using an umbrella sampling technique we define the starting configurations of the set of replicas for the next step. We test the method with the familiar 2-dimensional lattice of Ising magnets. Below the transition temperature there is a pronounced tendency for trapping in metastable ordered states.

Chemical Physics, 2007

We consider the computation of the surface tension of the fluid-fluid interface for the Widom-Row... more We consider the computation of the surface tension of the fluid-fluid interface for the Widom-Rowlinson [J. Chem. Phys. 52, 1670 (1970)] binary mixture from direct simulation of the inhomogeneous system. We make use of the standard mechanical route, in which the surface tension follows from the computation of the normal and tangential components of the pressure tensor of the system. In addition to the usual approach, which involves simulations of the inhomogeneous system in the canonical ensemble, we also consider the computation of the surface tension in an ensemble where the pressure perpendicular (normal) to the planar interface is kept fixed. Both approaches are seen to provide consistent values of the interfacial tension. The issue of the system-size dependence of the surface tension is addressed. In addition, simulations of the fluid-fluid coexistence properties of the mixture are performed in the semigrand canonical ensemble. Our results are compared with existing data of the Widom-Rowlinson mixture and are also examined in the light of the vapor-liquid equilibrium of the thermodynamically equivalent one-component penetrable sphere model.

Molecular Physics, 2003

Using extensive Monte Carlo simulations with both particle and cluster orientational moves, in co... more Using extensive Monte Carlo simulations with both particle and cluster orientational moves, in conjunction with finite size scaling and histogram reweighting techniques, we have determined the Curie temperature for two models of positionally frozen Heisenberg spin systems: a system with spatial correlations corresponding to a hard sphere fluid and a spatially random system. We find that the results for the positionally frozen hard sphere Heisenberg system are fairly similar to those previously obtained for the Heisenberg spin fluid and quantitatively agree with the mean field theory estimates. The random system undergoes the ferromagnetic transition at a higher temperature since the lack of core repulsion increases the spin correlations. In this case however the mean field theory overestimates by far the critical temperature.

European Physical Journal B, 2003

Physical Review E, 2007

In this work we investigate the phase behavior of a family of continuous bidimensional n-vector m... more In this work we investigate the phase behavior of a family of continuous bidimensional n-vector models ͑with n = 2, 3, and 4͒ using Monte Carlo simulation. In all cases we detect the presence of a defect mediated order-disorder transition of the Berezinskii-Kosterlitz-Thouless type with critical temperatures that decrease with the spin dimensionality. Coupled with the order-disorder transition a gas-liquid equilibrium is found at low temperatures. Here one observes that the stability region of the liquid phase shrinks with the growing spin dimensionality, in parallel with a decrease in magnitude of the angular averaged spin-spin interaction.