Gibbs ensemble Monte Carlo method

Simulating Phase Equilibria by the Gibbs Ensemble Monte Carlo Method... 

Lopes, J. N. C. Tildesley, D. J., Multiphase equilibria using the Gibbs ensemble Monte Carlo method, Mol. Phys. 1997, 92, 187-196... 

Kristof, T. Liszi, J., Application of a new Gibbs ensemble Monte Carlo method to site-site interaction model fluids, Mol. Phys. 1997, 90, 1031-1034... 

The phase equilibrium between a liquid and a gas can be computed by the Gibbs ensemble Monte Carlo method. We create two boxes, where the first box represents the dense phase and the second one represents the dilute phase. Each particle in the boxes experiences a Lennard-Jones potential from all the other particles. Three types of motion will be conducted at random the first one is particle translational movement in each box, the second one is moving a small volume from one box and adding to the other box, the third one is removing a particle from one box and inserting in the other box. After many such moves, the two boxes reach equilibrium with one another, with the same temperature and pressure, and we can compute their densities. 

Many computational studies of the permeation of small gas molecules through polymers have appeared, which were designed to analyze, on an atomic scale, diffusion mechanisms or to calculate the diffusion coefficient and the solubility parameters. Most of these studies have dealt with flexible polymer chains of relatively simple structure such as polyethylene, polypropylene, and poly-(isobutylene) [49,50,51,52,53], There are, however, a few reports on polymers consisting of stiff chains. For example, Mooney and MacElroy [54] studied the diffusion of small molecules in semicrystalline aromatic polymers and Cuthbert et al. [55] have calculated the Henry s law constant for a number of small molecules in polystyrene and studied the effect of box size on the calculated Henry s law constants. Most of these reports are limited to the calculation of solubility coefficients at a single temperature and in the zero-pressure limit. However, there are few reports on the calculation of solubilities at higher pressures, for example the reports by de Pablo et al. [56] on the calculation of solubilities of alkanes in polyethylene, by Abu-Shargh [53] on the calculation of solubility of propene in polypropylene, and by Lim et al. [47] on the sorption of methane and carbon dioxide in amorphous polyetherimide. In the former two cases, the authors have used Gibbs ensemble Monte Carlo method [41,57] to do the calculations, and in the latter case, the authors have used an equation-of-state method to describe the gas phase. 

The most striking news that one learns when studying vapor-liquid phenomena is that not only does the vapor need to nucleate a liquid droplet to condense, but that also the liquid needs to nucleate a gas bubble to evaporate [24]. On the theoretical side, the simulation is made easier because the vapor is relatively simple to handle, on the experimental side, vapor pressure measurements in vapor-liquid equilibrium are fairly easy to perform. The Gibbs ensemble Monte Carlo method (Section 9.8) can be applied to the vapor-liquid equilibrium with considerable success vapor pressure curves, second virial coefficients, and other equilibrium properties can be calculated by molecular simulation, and, remarkably, good results can apparently be obtained by highly accurate ab initio quantum mechanical potentials [25a] or by simple empirical potentials [25b]. 

Fig. 3.23 The Gibbs ensemble Monte Carlo simulation method uses one box for each of the two plwses. Three types < move are permitted translations within either box volume changes (keeping the total volume constant) and transfer a particle from one box to the other.

Another method of simulating chemical reactions is to separate the reaction and particle displacement steps. This kind of algorithm has been considered in Refs. 90, 153-156. In particular. Smith and Triska [153] have initiated a new route to simulate chemical equilibria in bulk systems. Their method, being in fact a generalization of the Gibbs ensemble Monte Carlo technique [157], has also been used to study chemical reactions at solid surfaces [90]. However, due to space limitations of the chapter, we have decided not to present these results. 

The first Monte Carlo study of osmotic pressure was carried out by Panagiotopoulos et al. [16], and a much more detailed study was subsequently carried out using a modified method by Murad et al. [17]. The technique is based on a generalization of the Gibbs-ensemble Monte Carlo (GEMC) method applied to membrane equihbria. The Gibbs ensemble method has been described in detail in many recent reports so we will only summarize the extension of the method to membrane equilibria here [17]. In the case of two phases separated by semi-permeable membranes... 

Kiyohara, K. Spyriouni, T. Gubbins, K. E. Panagiotopoulos, A. Z., Thermodynamic scaling Gibbs ensemble Monte Carlo a new method for determination of phase coexistence properties of fluid, Mol. Phys. 19%, 89, 965-974. 

Gibbs Ensemble Monte Carlo (GEMC) is an ingenious method introduced by Panagiotopoulos [72], which allows one to simulate the coexistence of liquid and vapor phases without having to deal with a physical interface between them. 

Direct Methods Simulate 2-phase system Gibbs ensemble Monte Carlo... 

At full saturation, corresponding to a vapor pressure of 0.044 bar at 300 K (determined by Gibbs Ensemble Monte Carlo [24] for SPC model), the total number of water molecules in the pores is around 10200, which corresponds to a density aroimd 0.90 g/cm, close to the density of SPC saturating water at 300 K (0.97 g/cm, by GEMC method). The discrepancy between both values is probably due to the slow convergence caused by a very low acceptance level of insertion of water molecules in liquid phase. 

Here we show that Eq. (8), together with the conditions of thermodynamic equilibrium for an isothermal adsorption system (equality of chemical potentials between the two phases), can be solved using the Gibbs ensemble Monte Carlo (GEMC) method in the modified form presented in the next section. 

Fig. 10. Schematic of the Gibbs ensemble Monte Carlo simulation method for calculation of phase equilibria of confined fluids [22].

I ic. 11. Relationship between the pore filling pressure and the pore width predicted by the modified Kelvin equation (MK). the Horvath-Kawazoe method (HK), density functional theory (DFT). and Gibbs ensemble Monte Carlo simulation (points) for nitrogen adsorption in carbon slit pores at 77 K [11]. 

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