Computational study of the structures and thermodynamic properties of ammonium chloride clusters using a parallel jump-walking approach

Monte Carlo methods were used to calculate heat capacities as functions of temperature for classical atomic clusters of aggregate sizes 25 ≤ N ≤ 60 that were bound by pairwise Lennard-Jones potentials. The parallel tempering method was used to overcome convergence difficulties due to quasiergodicity in the solid-liquid phase-change regions. All of the clusters studied had pronounced peaks in their heat capacity curves, most of which corresponded to their solid-liquid phase-change regions. The heat capacity peak height and location exhibited two general trends as functions of cluster size: for N = 25 to 36, the peak temperature slowly increased, while the peak height slowly decreased, disappearing by N = 37; for N = 30, a very small secondary peak at very low temperature emerged and quickly increased in size and temperature as N increased, becoming the dominant peak by N = 36. Superimposed on these general trends were smaller fluctuations in the peak heights that corresponded to "magic number" behavior, with local maxima found at N = 36, 39, 43, 46 and 49, and the largest peak found at N = 55. These magic numbers were a subset of the magic numbers found for other cluster properties, and can be largely understood in terms of the clusters' underlying geometries. Further insights into the melting behavior of these clusters were obtained from quench studies and by examining rms bond length fluctuations.

Section: Parallel Temperingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Magic number behavior for heat capacities of medium-sized classical Lennard-Jones clusters

Frantz

2001

“…In this method detailed balance is strictly satisfied only in the limit that the external distribution is of infinite size. In the second method, often called parallel J-walking, 31,32 the walks at each temperature are made in tandem on a parallel machine. Many processors, randomly initialized, are assigned to the jumping temperature, and each processor at the jumping temperature is used to donate a high temperature configuration to the low temperature walk sufficiently infrequently that the correlations in the Metropolis walk at inverse temperature ␤ J are broken.…”

Section: ͑8͒mentioning

confidence: 99%

Phase changes in 38-atom Lennard-Jones clusters. I. A parallel tempering study in the canonical ensemble

Neirotti

Calvo

Freeman

et al. 2000

Self Cite

259

266

The heat capacity and isomer distributions of the 38-atom Lennard-Jones cluster have been calculated in the canonical ensemble using parallel tempering Monte Carlo methods. A distinct region of temperature is identified that corresponds to equilibrium between the global minimum structure and the icosahedral basin of structures. This region of temperatures occurs below the melting peak of the heat capacity and is accompanied by a peak in the derivative of the heat capacity with temperature. Parallel tempering is shown to introduce correlations between results at different temperatures. A discussion is given that compares parallel tempering with other related approaches that ensure ergodic simulations.

“…Calculations of equilibrium properties when phase space is thus partitioned require methods that overcome quasiergodicity by enhanced barrier crossing. Many techniques have been proposed to address this problem, including the use of generalized ensembles such as multicanonical 1 or Tsallisian, 2,3 simulated tempering, 4 configurational 5 or force bias 6 Monte Carlo, or various versions of the jumpwalking [7][8][9][10][11] algorithm. Most of these techniques have been introduced for Monte Carlo ͑MC͒ simulations rather than molecular dynamics ͑MD͒ simulations.…”

Section: Introductionmentioning

confidence: 99%

Phase changes in 38-atom Lennard-Jones clusters. II. A parallel tempering study of equilibrium and dynamic properties in the molecular dynamics and microcanonical ensembles

Calvo

Neirotti

Freeman

et al. 2000

Self Cite

190

162

We study the 38-atom Lennard-Jones cluster with parallel tempering Monte Carlo methods in the microcanonical and molecular dynamics ensembles. A new Monte Carlo algorithm is presented that samples rigorously the molecular dynamics ensemble for a system at constant total energy, linear and angular momenta. By combining the parallel tempering technique with molecular dynamics methods, we develop a hybrid method to overcome quasiergodicity and to extract both equilibrium and dynamical properties from Monte Carlo and molecular dynamics simulations. Several thermodynamic, structural, and dynamical properties are investigated for LJ 38 , including the caloric curve, the diffusion constant and the largest Lyapunov exponent. The importance of insuring ergodicity in molecular dynamics simulations is illustrated by comparing the results of ergodic simulations with earlier molecular dynamics simulations.