Gibbs Ensemble Monte Carlo Research Articles

Molecular simulation of liquid-liquid equilibria for Lennard-Jones fluids

Abstract Guo M.X., Li Y.G., Li Z.C. and Lu J.F., 1994. Molecular simulation of liquid-liquid equilibria for Lennard-Jones fluids. Fluid Phase Equilibria , 98: 129-139. In this work, the liquid-liquid phase behavior of Lennard-Jones fluids was simulated by a Gibbs ensemble Monte Carlo (GEMC) method. To improve the convergence of simulation on such high-density systems, an “Internal Identity Interchange” move was proposed and added to the original GEMC method. Five binary mixtures were designed to examine the effects of various molecular parameters on the immiscibility of the two coexistence liquid phases. To study the influences of the two cross-molecular parameters (ϵ 12 and σ 12 ) on the immiscibility of the coexistence liquid phases, the Lorentz-Berthelot combining rules were linearly scaled through multiplication by two constants.

Vapor-liquid equilibria of model alkanes

In this communication, the results of computer simulations of the vapor-liquid equilibria of n-pentane and n-octane are reported. We have used a novel Monte Carlo technique to simulate directly phase equilibria of chain molecules. The calculations were performed for a united-atom model using the Lennard-Jones parameter set of Jorgensen and co-workers.' Comparison of the calculated phase diagrams with experimental data shows very good agreement, in particular for the shorter alkane. Computer simulations have contributed to the general understanding of the behavior of chain molecules. The success of a simulation depends to a large extent on the judicious choice of molecular potentials. An example of a popular potential field is the OPLS model (optimized potentials for liquid simulations) which has been used to study systems ranging from simple alkanes'J and surf act ants'^^ to polymers5 and proteins.6 It is important to realize that the OPLS model has been fitted to liquid properties of small molecules at room temperature. In practice, however, these models are used beyond the conditions at which they have been fitted. An ideal test of model potentials is the comparison with experimental phase diagrams. Unfortunately, determining a phase diagram is an extremely difficult task,' in particular, for chain molecules. Over the last few years there has been an impressive improvement in the development of simulation techniques for phase equilibria. Most noticeable is the introduction of the GibbsEnsemble Monte Carlo (GEMC) technique by Panagiotopoulos.*.9 In the GEMC technique two separate boxes are utilized. Besides the conventional random displacement of particles, additional types of Monte Carlo moves are applied to ensure that the two boxes are in equilibrium, Le., have equal temperature, pressure, and chemical potential. The particular advantage of the Gibbs ensemble is that if conditions are such that the system wants to phase separate, the simulation yields a vapor phase in one box and a liquid phase in the other. As a result, the coexistence properties can be determined directly. One of the Monte Carlo steps involves the swapping of a molecule from the vapor to the liquid box and vice uersa. While this step is relatively easy for systems containing atoms or small molecules, it is virtually impossible to insert chain molecules into their liquid phase using the conventional techniques. To enhance the sampling of chain insertions we have used a novel technique based on the configurational-bias Monte Carlo method (CBMC).loJ1 The random insertion is replaced with a scheme in which a chain molecule is inserted atom by atom such that conformations with favorable

Computer Simulation Study of Adsorption, Isosteric Heat and Phase Transitions of Methane on Graphite

AbstractWe report Canonical (NVTMC), Grand Canonical (GOMC) and Gibbs Ensemble Monte Carlo (GEMC) simulations for adsorption of methane on graphite. Lennard-Jones (LJ) potentials are used for the intermolecular interactions, and both structured and structureless (10–4–3) solid-fluid potentials are considered. Several sets of methane-methane L.I parameters have been used in the literature, and we compare results obtained with these sets throughout our simulations. The adsorption isotherm and isosteric heat curve are obtained at 77.5 K and found in good agreement with experiments. The commensurateincommensurate transition (CIT) of methane on a graphite substrate with periodically varying adsorbate-adsorbent potential at 40.0 K is studied and is in qualitative agreement with experiment. The effect of varying the corrugation of the fluid-wall potential on the commensurate and incommensurate phases is explored. The GEMC simulations have been carried out to study the vapor-liquid equilibrium (VLE) of a two-dimensional (2D) LJ fluid with system sizes up to 3000 particles. The effect of system sizes on the critical behavior is investigated. The GEMC method has also been successfully applied to study the VLE in 2D adsorbed films for the first time.

Comment on: Molecular simulation of water along the liquid–vapor coexistence curve from 25 °C to the critical point

In an earlier paper [J. Chem. Phys. 93, 7355 (1990)], de Pablo et al. reported Gibbs ensemble Monte Carlo (GEMC) simulations of SPC water with long-range intermolecular interactions computed using the Ewald summation method. The coexistence curve of SPC water was obtained from 25 to 300 °C. We report new simulations with a slight modification of the SPC potential to reflect the lower effective dipole moment in the vapor phase. The vapor–liquid coexistence curve is found to agree better with experiment.