Gibbs Ensemble Monte Carlo Method Research Articles

Molecular Simulation Studies on the Vapor-Liquid Equilibrium of CO2 + 3,3,3-Trifluoropropene (R1243zf) Binary Mixtures.

As fourth-generation refrigerants with great development prospects, hydrofluoroolefins (HFOs) can be mixed with other refrigerants, such as carbon dioxide (CO2), to form refrigerant mixtures with low global warming potential (GWP) and zero ozone depleting potential (ODP) while retaining the advantages of each component. Refrigerants can work together to achieve complementary benefits. Combinations of CO2 and HFOs can strengthen the thermodynamic properties of CO2 while inhibiting the flammability of HFOs. At present, relatively few studies have been conducted on the CO2 + 3,3,3-trifluoropropene (R1243zf) mixture. Besides experimental approaches, molecular simulation has grown in importance as a way to determine thermodynamic and transport properties in recent years. In this study, the Gibbs Ensemble Monte Carlo (GEMC) method was used to simulate the vapor-liquid equilibrium (VLE) properties of the CO2 + R1243zf binary mixture in the temperature range of 273.15 to 313.15 K, in which the three-site rigid TraPPE force field and a fully flexible transferable all-atom force field were selected to describe CO2 and R1243zf, respectively. By comparing the GEMC simulation results with the experimental data, it was found that the average deviation of pressure is 2.33%, the average deviation of liquid phase molar fraction Δx is 0.0099, and the mean deviation of gaseous phase molar fraction Δy is 0.0204. The simulation results accord well with the experimental data and the fitting data of the correlation model in the literature, indicating that the molecular models used for CO2 and R1243zf can reliably predict the VLE properties. Finally, the critical parameters of the mixture at a temperature of 313.15 K were predicted, and the radial distribution functions (RDFs) of pure R1243zf and the mixture at 273.15 K and 3.5 MPa were calculated and analyzed by molecular dynamics (MD) simulation.

A study on the vapor–liquid equilibria of vinyl acrylate-acetic acid and vinyl acetate-acetic acid with GEMC method

Vinyl acetate (VAc) is one of the widely utilized industrial organic raw materials. In the acetoxylation of ethylene to vinyl acetate, there is a by-product vinyl acrylate. However, the vapor–liquid phase equilibrium data of this component with acetic acid and vinyl acetate have not been reported. Here, by Density Functional Theory calculations, we fitted the TraPPE-UA force field parameters of vinyl acrylate. The vapor–liquid phase equilibrium data of the vinyl acrylate-acetate and vinyl acrylate-vinyl acetate binary systems were calculated using the Gibbs Ensemble Monte Carlo (GEMC) method. The calculated phase equilibrium data were utilized to fit the binary interaction parameters of the NRTL equations for the binary systems which were used for the separation process simulation of the vinyl acrylate, acetic acid and vinyl acetate systems. It was verified that the TraPPE-UA force field parameters fitted in this paper have high accuracy. The results clarify the concentration distribution of vinyl acrylate gas–liquid phase components in the distillation column, which builds the foundation for the development of the separation process.

Monte Carlo simulations and molecular discrete perturbation theory of multipolar oblate Kihara fluids

The vapor-liquid coexistence of oblate Kihara fluids embedding both point dipole or quadrupole moments has been determined within the framework of the Gibbs ensemble Monte Carlo method and predicted by the molecular discrete perturbation theory. For dipolar models, we explored both axial and non-axial orientations of the dipole moment with respect to the molecular axis. The discrepancies between theoretical and simulations results are discussed in terms of both the reliability of the approximations in which the free energy terms in the perturbation expansion for molecular fluids are based and of the aggregation features found in the structural distribution functions of the liquid phases close to the coexistence boundary, more prominent for highly anisotropic fluids. As a last output of the study, we found a non-monotonous behavior of the critical properties for dipolar models that seems to be wedded to that structuring/aggregation effects, mostly induced by the strongly oriented multipolar interactions.

Simulation of adsorption and release of doxepin onto ZIF-8 including in vitro cellular toxicity and viability

Doxepin hydrochloride is an antidepressant drug commonly used to treat mild to moderate depression. However, this drug has poor oral bioavailability, with the percentage of drug that can enter the bloodstream being weak and variable. The present study seeks to investigate the ZIF-8 nanoparticles for doxepin delivery and to assess this drug uptake and release in ZIF-8 frameworks using the Gibbs-ensemble Monte Carlo method in the Simulated Materials Studio. This study also adopted a model for showing surface adsorbent (ZIF-8S) capabilities. ZIF-8 nano-frameworks have many advantages such as easy flowing through capillaries leading to a satisfactory drug release into the systemic circulation and preventing high plasma concentration. Additionally, the study examined the diffusion of drug molecules using the simulated molecular dynamics with regard to the structural flexibility of the MOF. Proper loading of 8.6% and slow diffusion of 7.353 × 10−6 cm2/s doxepin molecules from ZIF-8 pores confirmed that this material could be recommended as an effective carrier for doxepin. Furthermore, the fixed ZIF-8S model is also used by keeping the opening of cavities fixed, thereby attaining a higher loading amount 12%. Moreover, since ethanol has been applied as a suitable solvent in many biological systems, of adsorption, diffusion processes are also repeated in this solvent. Thus, presence of such adsorption sites as zinc ions may facilitate the interaction of the framework with large molecules to provide desirable loading and diffusion amounts. Furthermore, more adsorption can be reached with less energy at the surface of the ZIF-8 in the presence of ethanol, which is due to the greater adsorption of solvent molecules. Finally, to ascertain the in vitro cellular toxicity and viability of the prepared nanoparticles, they are treated with PC3 human prostate cancer cells and the average growth curves are monitored via MTT assay. This study confirmed a decrease in the MTT signal compared to the untreated cells, so ZIF-8 can reduce the metabolic activity of the performed cancer cells. This study shows that this nanomaterial can be achieved as a vehicle for delivery of doxepin.

Effect of water on high-pressure ternary phase equilibria of CO2 + H2O + alkanolamine based ionic liquid

Some ethanolamine or hydroxylammonium based ionic liquids have shown great potential in CO2 separation processes. However, not much is known about the influence of water on the CO2 solubility of these liquids. In this paper, we study the high-pressure ternary liquid-vapor phase equilibria. The systems were composed of carbon dioxide, water, and an alkanolamine based IL: N-methyl-2-hydroxyethylammonium propionate [m-2HEA][Pr]. We studied systems whose water contents varied from 6.7% to 52.5%, temperatures from 313 K to 353 K, and pressures from 0.6 MPa up to 22 MPa. The phase equilibria experiments showed that the amount of ionic liquid in the mixture controls the solubilization of CO2 and is not significantly affected by changes in the water content within the studied range. These data were simulated from a molecular point of view using the Gibbs ensemble Monte Carlo method. A force field was estimated to describe the Ionic Liquid (IL) density at a wide range of water content values using the isothermal−isobaric Monte Carlo (NPT-MC). The calculated densities presented a good agreement with experimental data, indicating that the proposed force field parameters are suitable to describe the densities of the IL mixtures and, therefore, can be used to simulate the ternary phase equilibria. The software package CassandraV1.2 was employed to simulate the ternary phase equilibria, resulting in reasonably low deviations when compared to the experimental data. However, the predicted solubility of carbon dioxide in N-methyl-2-hydroxyethylammonium propionate [m-2HEA][Pr] was slightly higher when compared with the experimental values.

Coexistence calculation using the isothermal-isochoric integration method

In this work, the isothermal-isochoric integration (ITIC) method is demonstrated as a viable method for vapor-liquid coexistence calculation by molecular simulation. Several tests are carried out to validate the method. The first group of tests utilizes self-consistent NIST REFPROP values to demonstrate that, in the absence of simulation uncertainties, the ITIC method yields coexistence values with less than 1% deviation for reduced temperatures of less than 0.85. The impact of various simulation specifications are then compared. Following our recommended simulation methodology, consistent results are achieved between the ITIC method, Gibbs Ensemble Monte Carlo (GEMC) method, and Grand Canonical Monte Carlo (GCMC) method for reduced temperatures of 0.6–0.85. The ITIC method proves to be much more effective compared to GEMC and GCMC methods for vapor-liquid coexistence calculations at reduced temperatures of 0.45–0.6, which are important for practical applications. It is shown that computational efficiency is often served best by applying the ITIC method for the entire temperature range rather than applying Monte Carlo (MC) methods for part of the range. Furthermore, the ITIC method lends itself to application with molecular dynamics (MD) as well as MC, advancing the prospect of simulation results that are quantitatively consistent across software platforms.

Integrated QMMM and Monte Carlo methods for analysis of adsorptive interactions between goethite cluster, carbon nanotubes, and arsenate

AbstractIron‐immobilized nanoporous carbon is a well‐known adsorbent used in treating arsenic‐contaminated waters. In this contribution, we present findings on the adsorptive interactions and dynamics of arsenate–goethite cluster ([FeO(OH)]6) with carbon nanotubes (CNTs) using hybridized quantum mechanics/molecular mechanics (QMMM) calculations. The CNTs adsorption mechanism is of interest since a better understanding of the fundamental interactions between arsenate, goethite, and carbon surfaces would translate to advances in CNT‐based adsorbent production and utilization. Novel applications of general amber force field (GAFF) and isobaric‐isothermal Gibbs ensemble Monte Carlo (NpT‐GEMC) methods are described. By the abovementioned methods, we postulate that the [FeO(OH)]6/CNT‐2.3 (diameter 2.3 nm ‐ mesoporous) system enhances the qualitative (i.e., improved chemisorption) rather than the quantitative adsorptive aspect (i.e., total ions adsorbed) in comparison to the [FeO(OH)]6/CNT‐1.6 (diameter 1.6 nm ‐ microporous) system.

Vapor Liquid Equilibria for Acetic Acid–Acetaldehyde–Crotonaldehyde System: Gibbs Ensemble Molecular Simulation for Pure Components and Binary Systems and NRTL Model Prediction for Ternary System

The vapor–liquid equilibrium (VLE) for the system acetic acid, acetaldehyde, and crotonaldehyde was investigated by combining the Gibbs ensemble Monte Carlo (GEMC) method and activity coefficient models prediction. The VLE of above pure components and their binary systems were first studied using GEMC method. Based on the strong association characteristics of acetic acid, the revised TraPPE-UA-D force field was developed with quantum chemistry methods for the VLE calculation of the acetic acid system. The original and mixed TraPPE-UA force field was introduced for the VLE calculation of the acetaldehyde and crotonaldehyde system, respectively. The results showed that the predictions agreed fairly well with the average relative deviations of the saturated densities smaller than 4.60%. Moreover, the binary VLE diagram was calculated using the above force fields. The simulation results of the mole composition were in good agreement with the experimental values, which verified the high simulation accuracy of ...

Chemical potentials of water, methanol, carbon dioxide and hydrogen sulphide at low temperatures using continuous fractional component Gibbs ensemble Monte Carlo

Chemical potentials of coexisting gas and liquid phases for water, methanol, hydrogen sulphide and carbon dioxide for the temperature range K to K are computed using two different methodologies: (1) Widom’s test particle insertion (WTPI) method in the conventional Gibbs Ensemble (GE), and (2) the Continuous Fractional Component Gibbs Ensemble Monte Carlo (CFCGE MC) method. It is shown that the WTPI method fails to accurately compute the chemical potentials of water and methanol in the liquid phase at low temperatures, while accurate chemical potentials in the liquid phase are computed using CFCGE MC method. For the CFCGE MC method, the statistical uncertainty for computed chemical potentials of water and methanol in the liquid phase are considerably smaller compared to the WTPI method. For the water models considered in this study (SPC, TIP3P-EW, TIP4P-EW, TIP5P-EW), computed excess chemical potentials based on three-site models are in better agreement with the chemical potentials computed from an empirical equation of state from the NIST database. For water, orientational biasing is applied during test particle insertion to check whether certain orientations of test particle are energetically unfavourable. A two-dimensional Overlapping Distribution Method (ODM) in the NVT ensemble is derived for this purpose. It is shown that failure of the WTPI method for systems with a strong hydrogen bonding network does not depend on orientation of the test molecule in that system. For all systems in this study, the WTPI method breaks down when the void fraction of the system drops below approximately 0.50.

Gibbs ensemble Monte Carlo with solvent repacking: phase coexistence of size–asymmetrical binary Lennard-Jones mixtures

We describe a Monte Carlo method for simulation of vapour–liquid phase coexistence in size-asymmetrical Lennard–Jones (LJ) binary mixtures. The method incorporates the Solvent Repacking Monte Carlo (SRMC) approach, which offers efficient trial moves for the exchange of a large particle for several small particles, into the Gibbs Ensemble Monte Carlo (GEMC) method. SRMC yields a significant efficiency improvement in simulation of dilute large species mixtures at low temperature compared to the original GEMC method with identity exchange moves. Vapor–liquid phase diagrams are reported for LJ mixtures with a diameter ratio of 1:2 with well depth ratios and 2, producing spindle-type, azeotrope and closed loop types of phase diagram, respectively.

Monte Carlo simulations of vapour–liquid phase equilibrium and microstructure for the system containing azeotropes

Phase equilibrium data of the mixtures including alcohols, esters and organic acids are of first interest particularly to design and optimise biodiesel production and reactive distillation processes. In this work, vapour–liquid phase equilibrium of these systems was simulated at low pressure using Gibbs ensemble Monte Carlo method. All Lennard–Jones parameters of pseudo-atoms involved in the systems were derived from previous parametrisations of TraPPE-UA force field. The Fourier coefficients of dihedrals encountered in ethyl acetate molecule have been obtained from the quantum calculations. Using this force field, temperature-composition diagrams are well reproduced for ethyl acetate + ethanol, ethyl acetate + methanol at 70.00 kPa and ethyl acetate + acetic acid mixtures at 77.33 kPa. The transferability of this force field to mixtures in these systems is noticeable. Analysis of the microstructure for the ethyl acetate + ethanol and ethyl acetate + acetic acid mixtures was presented. We found that the hydrogen bond networks consist of autoassociation and cross-association and autoassociation occupies the main position as compared with cross association in the ethyl acetate + ethanol mixture. OCHAc–HHAc and OCEtOAc–HHAc hydrogen bond interactions play a significant role in the phase behaviours or structures of ethyl acetate + acetic acid mixture.

A Monte Carlo simulation study to predict the solubility of H2S in ionic liquids with 1-butyl-3-methylimidazolium ([C4mim+]) cation and tetrafluoroborate ([BF4−]), hexaflorophosphate ([PF6−]) and bis(trifluoromethanesulfonyl)amide ([Tf2N−]) anions

The solubilities of H2S in ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim+][BF4−]), 1-butyl-3-methylimidazolium hexaflorophosphate ([C4mim+][PF6−]) and -butyl-3-methylimidazolium tetrafluoroborate bis(trifluoromethanesulphonyl)amide ([C4mim+][Tf2N−]) are predicted using isothermal–isobaric Gibbs ensemble Monte Carlo method (NPT-GEMC) at temperatures ranging from 333 to 453 K and pressure up to 20 bar. The low pressure points (up to 3 bar) of the absorption isotherms are fitted to a straight line to get a rough estimation of the Henry’s law constants. The van’t Hoff relationship is used to estimate the partial molar enthalpy of the absorption. The obtained results are in a good agreement with Jou and Mather [18], and Jalili et al. [13]. When comparing the solubility in ILs, it is found that H2S solubility is highest for [C4mim+][Tf2N−], followed by [C4mim+][PF6−]. The lowest solubility is observed in [C4mim+][BF4−]. The highest solubility in [C4mim+][Tf2N−] is consistent with Jalili et al. [13]. However, their results indicate slightly higher solubility in [C4mim+][BF4−] than in [C4mim+][PF6−], and do not agree with the predictions. Upon absorption, the molar volumes of the mixtures decrease linearly, showing only small changes in volume. The effect of H2S absorption on ILs is further studied by calculating the radial distribution functions between the ions. The results indicate that the solute molecules accommodate themselves in the cavities without significantly disturbing the ionic arrangement of the ions, similar to CO2 absorption in ILs. The spatial distribution functions show similar spatial distribution for H2S around cation in all of the studied ILs, whereas the distribution around anion depends on the shape and flexibility of the anion. The mechanism of H2S absorption is studied by computing the van der Waals (VDW) and electrostatic (ELEC) energies. It is observed that the solubility of H2S in the studied ILs is primarily controlled by VDW interaction. When comparing the interaction of H2S with the ions, it is found that solute molecules interact with cations mainly due to VDW interaction. Both VDW and ELEC energies contribute in the interaction between H2S and anions.

A new kinetic Monte Carlo scheme with Gibbs ensemble to determine vapour–liquid equilibria

We present a new kinetic Monte Carlo scheme, as an alternative to the Gibbs ensemble Monte Carlo (GEMC) method, to determine vapour–liquid equilibria using a canonical ensemble in a system composed of two boxes. To illustrate the method, we have tested it with two systems: (1) argon over a range of temperatures from below the triple point to close to the critical point; (2) methane and ethane mixtures of various compositions at 180 K. The advantage of the new scheme is that chemical potentials of all components are accurately determined in both boxes. In particular, the chemical potential in the liquid box is determined much more accurately than with the Widom method employed in conventional GEMC simulations.

Vapor Pressure of Aqueous Solutions of Electrolytes Reproduced with Coarse-Grained Models without Electrostatics.

The vapor pressure of water is a key property in a large class of applications from the design of membranes for fuel cells and separations to the prediction of the mixing state of atmospheric aerosols. Molecular simulations have been used to compute vapor pressures, and a few studies on liquid mixtures and solutions have been reported on the basis of the Gibbs Ensemble Monte Carlo method in combination with atomistic force fields. These simulations are costly, making them impractical for the prediction of the vapor pressure of complex materials. The goal of the present work is twofold: (1) to demonstrate the use of the grand canonical screening approach ( Factorovich , M. H. J. Chem. Phys. 2014 , 140 , 064111 ) to compute the vapor pressure of solutions and to extend the methodology for the treatment of systems without a liquid-vapor interface and (2) to investigate the ability of computationally efficient high-resolution coarse-grained models based on the mW monatomic water potential and ions described exclusively with short-range interactions to reproduce the relative vapor pressure of aqueous solutions. We find that coarse-grained models of LiCl and NaCl solutions faithfully reproduce the experimental relative pressures up to high salt concentrations, despite the inability of these models to predict cohesive energies of the solutions or the salts. A thermodynamic analysis reveals that the coarse-grained models achieve the experimental activity coefficients of water in solution through a compensation of severely underestimated hydration and vaporization free energies of the salts. Our results suggest that coarse-grained models developed to replicate the hydration structure and the effective ion-ion attraction in solution may lead to this compensation. Moreover, they suggest an avenue for the design of coarse-grained models that accurately reproduce the activity coefficients of solutions.

Accurate and precise determination of critical properties from Gibbs ensemble Monte Carlo simulations.

Since the seminal paper by Panagiotopoulos [Mol. Phys. 61, 813 (1997)], the Gibbs ensemble Monte Carlo (GEMC) method has been the most popular particle-based simulation approach for the computation of vapor-liquid phase equilibria. However, the validity of GEMC simulations in the near-critical region has been questioned because rigorous finite-size scaling approaches cannot be applied to simulations with fluctuating volume. Valleau [Mol. Simul. 29, 627 (2003)] has argued that GEMC simulations would lead to a spurious overestimation of the critical temperature. More recently, Patel et al. [J. Chem. Phys. 134, 024101 (2011)] opined that the use of analytical tail corrections would be problematic in the near-critical region. To address these issues, we perform extensive GEMC simulations for Lennard-Jones particles in the near-critical region varying the system size, the overall system density, and the cutoff distance. For a system with N = 5500 particles, potential truncation at 8σ and analytical tail corrections, an extrapolation of GEMC simulation data at temperatures in the range from 1.27 to 1.305 yields T(c) = 1.3128 ± 0.0016, ρ(c) = 0.316 ± 0.004, and p(c) = 0.1274 ± 0.0013 in excellent agreement with the thermodynamic limit determined by Potoff and Panagiotopoulos [J. Chem. Phys. 109, 10914 (1998)] using grand canonical Monte Carlo simulations and finite-size scaling. Critical properties estimated using GEMC simulations with different overall system densities (0.296 ≤ ρ(t) ≤ 0.336) agree to within the statistical uncertainties. For simulations with tail corrections, data obtained using r(cut) = 3.5σ yield T(c) and p(c) that are higher by 0.2% and 1.4% than simulations with r(cut) = 5 and 8σ but still with overlapping 95% confidence intervals. In contrast, GEMC simulations with a truncated and shifted potential show that r(cut) = 8σ is insufficient to obtain accurate results. Additional GEMC simulations for hard-core square-well particles with various ranges of the attractive well and for n-decane molecules represented by the TraPPE force field yield data that support the trends observed for Lennard-Jones particles. The finite-size dependence of the critical properties obtained from GEMC simulations is significantly smaller than those from grand-canonical ensemble simulations. Thus, when resources are not available for a rigorous finite-size scaling study, GEMC simulations provide a straightforward route to determine fairly accurate critical properties using relatively small system sizes.

A Monte Carlo Simulation Study To Predict the Solubility of Carbon Dioxide, Hydrogen, and Their Mixture in the Ionic Liquids 1-Alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Cnmim+][Tf2N–], n = 4, 6)

Ionic liquids are being investigated as potential solvents for pre- and post-combustion CO2 capture. In pre-combustion applications, knowledge of the selectivity of CO2 over H2 in high-pressure and high-temperature gas mixtures is crucial. In the present work, the solubility of carbon dioxide, hydrogen, and their mixture in the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Cnmim+][Tf2N–], n = 4, 6) is computed using the isothermal–isobaric Gibbs ensemble Monte Carlo (NPT-GEMC) method. The predicted solubility for CO2 and H2 in 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim+][Tf2N–]) and 1-n-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C6mim+][Tf2N–]) are compared with experimental results. It is found that the predicted absorption isotherms for pure gases in the studied ionic liquids (ILs) agree reasonably well with the available experimental results. The Henry’s Law constants, partial molar enthalpies of absorption, partial molar volumes, and the percentage change in the volumes of the IL phase upon absorption of CO2 and H2 in [C4mim+][Tf2N–] and [C6mim+][Tf2N–] are estimated at different temperatures and are compared with the experimental data. The results indicate that the solubility of CO2 decreases with increasing temperature, whereas, for H2, it slightly increases with increasing temperature, which is consistent with most previous experimental data. Mixed-gas (CO2/H2) experiments are much harder to conduct than pure gas experiments, so it is usually assumed that absorption selectivity is ideal. To test this, mixed gas simulations were also carried out using the NPT-GEMC method. The simulations indicate that the solubility selectivity of CO2 over H2 in [C4mim+][Tf2N–] and [C6mim+][Tf2N–] varies, depending on the vapor phase composition. The selectivities are compared with the Henry’s Law constant ratios and ideal selectivities that are obtained from pure gas isotherms for CO2 and H2 in both ionic liquids. Both computed selectivities and ideal selectivities are close and are different from Henry’s Law constant ratios at 333 K. The results indicate no enhancement in the solubility of H2, because of the presence of CO2 in the IL phase. The selectivity decreases with increasing temperature, which is consistent with the simulation study of Shi et al. [J. Phys. Chem. B 2010, 114, 6531]. This is due to the fact that CO2 solubility decreases with increasing temperature, whereas H2 solubility increases.

Quantum Gibbs ensemble Monte Carlo.

We present a path integral Monte Carlo method which is the full quantum analogue of the Gibbs ensemble Monte Carlo method of Panagiotopoulos to study the gas-liquid coexistence line of a classical fluid. Unlike previous extensions of Gibbs ensemble Monte Carlo to include quantum effects, our scheme is viable even for systems with strong quantum delocalization in the degenerate regime of temperature. This is demonstrated by an illustrative application to the gas-superfluid transition of (4)He in two dimensions.

Influence of simulation protocols on the efficiency of Gibbs ensemble Monte Carlo simulations

The Gibbs ensemble Monte Carlo (GEMC) method is a versatile approach for the prediction of fluid phase equilibria from particle-based simulations. For a one-component system, a GEMC simulation utilises two separate simulation boxes for the vapour and the liquid phases and a significant fraction of the computational effort is expended on special trial moves that transfer (swap) particles and exchange volume between the two boxes. The user needs to specify the frequency of swap and volume moves and the overall volume that controls the phase ratio. In this study, the efficiency of GEMC simulation protocols that yield three different frequencies of accepted swap and volume moves and three different phase ratios is assessed for the computation of the saturated vapour pressure and liquid density of n-octane and water at three reduced temperatures. Differences in the simulation efficiency of up to an order of magnitude are observed, and recommendations are made for suitable GEMC simulation protocols.

Computer simulation of liquid-vapor coexistence of confined quantum fluids

The liquid-vapor coexistence (LV) of bulk and confined quantum fluids has been studied by Monte Carlo computer simulation for particles interacting via a semiclassical effective pair potential Veff(r) = VLJ + VQ, where VLJ is the Lennard-Jones 12-6 potential (LJ) and VQ is the first-order Wigner-Kirkwood (WK-1) quantum potential, that depends on β = 1∕kT and de Boer's quantumness parameter Λ=h/σ√mε, where k and h are the Boltzmann's and Planck's constants, respectively, m is the particle's mass, T is the temperature of the system, and σ and ε are the LJ potential parameters. The non-conformal properties of the system of particles interacting via the effective pair potential Veff(r) are due to Λ, since the LV phase diagram is modified by varying Λ. We found that the WK-1 system gives an accurate description of the LV coexistence for bulk phases of several quantum fluids, obtained by the Gibbs Ensemble Monte Carlo method (GEMC). Confinement effects were introduced using the Canonical Ensemble (NVT) to simulate quantum fluids contained within parallel hard walls separated by a distance Lp, within the range 2σ ≤ Lp ≤ 6σ. The critical temperature of the system is reduced by decreasing Lp and increasing Λ, and the liquid-vapor transition is not longer observed for Lp∕σ < 2, in contrast to what has been observed for the classical system.

Constant Pressure Path Integral Gibbs Ensemble Monte Carlo Method

We present the implementation of a real-space constant pressure path integral Gibbs ensemble Monte Carlo (CP-PIGEMC) method for the simulation of one-component fluid consists of distinguishable quantum particles (henceforth referred to as Boltzmannons) in an external potential field at finite temperatures. We apply this simulation method to study the para-H2 adsorption in NaX zeolite at 77 K and pressures up to 100 bar. We present a new set of effective solid-fluid parameters optimized for path integral simulations of hydrogen isotope adsorption and separation in synthetic zeolites. The agreement among CP-PIGEMC, experiment, and the path integral grand canonical Monte Carlo method (PIGCMC) is very good, even at high pressures. CP-PIGEMC is a particularly useful method for simulation of one-component quantum fluid composed of Boltzmannons at finite temperatures, when the chemical potential is difficult to measure or calculate explicitly.