Histogram-reweighting Monte Carlo Simulations Research Articles

Optimized Mie Potentials for Phase Equilibria: Application to Branched Alkanes

A transferable united-atom (UA) force field based on Mie potentials is presented for branched alkanes. The performance of the optimized Mie potential parameters is assessed for 32 branched isomers of butane, pentane, hexane, heptane, and octane using grand canonical histogram-reweighting Monte Carlo simulations. For each compound, vapor–liquid-coexistence curves, vapor pressures, heats of vaporization, critical properties, and normal boiling points are predicted and compared to experiment. Experimental saturated liquid densities and critical temperatures are reproduced with a median absolute average error of 0.6%, while vapor pressures are reproduced with a median absolute average error of 2.2%. Calculations performed with the TraPPE and NERD force fields produce median absolute average errors for saturated liquid densities and vapor pressures of 1.3–1.8% and 14.3–23.5%, respectively. Binary phase diagrams predicted by the Mie potentials for argon+neopentane, methane+neopentane, and ethane+isobutane are i...

Optimised Mie potentials for phase equilibria: application to alkynes

ABSTRACTA transferable united-atom force field, based on Mie potentials, is presented for alkynes. The performance of the optimised Mie potential parameters is assessed for 1-alkynes and 2-alkynes using grand canonical histogram-reweighting Monte Carlo simulations. For each compound, vapour–liquid coexistence curves, vapour pressures, heats of vapourisation, critical properties and normal boiling points are predicted and compared to experiment. Experimental saturated liquid densities are reproduced to within 2% average absolute deviation (AAD), except for 1-hexyne, which are reproduced with 3% AAD. Experimental saturated vapour pressures are reproduced to within 3% AAD, except for 1-pentyne, 2-pentyne and 2-hexyne, where deviations from experiment of up to 20% AAD were observed. Binary phase diagrams, predicted from Gibbs ensemble Monte Carlo simulations, for propane + propyne, propene + propyne and propadiene + propyne, are in close agreement with experiment.

Prediction of Radon-222 Phase Behavior by Monte Carlo Simulation

Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine saturated liquid and vapor densities, vapor pressures, heats of vaporization, and compressibility factors for radon-222 from the normal boiling point to the critical point. An optimized intermolecular potential is developed by fitting parameters to reproduce experimental vapor pressures and the critical temperature. Vapor pressures are reproduced by simulation to within 2.2% of experiment, while the critical temperature and normal boiling point are reproduced exactly within the combined uncertainty of simulation and experimental data. The predictions of simulation are used to evaluate the reliability of reported experimental liquid densities, and those predicted by equations of state. While the predictions of simulation are in agreement with equations of state for the liquid density, significant differences are observed between the predictions of simulation and experiment. These results suggest that previously reported experimental liquid densities for radon-222 may be erroneous.

Optimized Mie potentials for phase equilibria: Application to noble gases and their mixtures with n-alkanes.

Transferrable force fields, based on n-6 Mie potentials, are presented for noble gases. By tuning the repulsive exponent, ni, it is possible to simultaneously reproduce experimental saturated liquid densities and vapor pressures with high accuracy, from the normal boiling point to the critical point. Vapor-liquid coexistence curves for pure fluids are calculated using histogram reweighting Monte Carlo simulations in the grand canonical ensemble. For all noble gases, saturated liquid densities and vapor pressures are reproduced to within 1% and 4% of experiment, respectively. Radial distribution functions, extracted from NVT and NPT Monte Carlo simulations, are in similarly excellent agreement with experimental data. The transferability of the optimized force fields is assessed through calculations of binary mixture vapor-liquid equilibria. These mixtures include argon + krypton, krypton + xenon, methane + krypton, methane + xenon, krypton + ethane, and xenon + ethane. For all mixtures, excellent agreement with experiment is achieved without the introduction of any binary interaction parameters or multi-body interactions.

Monte Carlo predictions of phase equilibria and structure for dimethyl ether + sulfur dioxide and dimethyl ether + carbon dioxide

A new force field for dimethyl ether (DME) based on the Lennard-Jones (LJ) 12-6 plus point charge functional form is presented in this work. This force field reproduces experimental saturated liquid and vapor densities, vapor pressures, heats of vaporization, and critical properties to within the statistical uncertainty of the combined experimental and simulation measurements for temperatures between the normal boiling and critical point. Critical parameters and normal boiling point are predicted to within 0.1% of experiment. This force field is used in grand canonical histogram reweighting Monte Carlo simulations to predict the pressure composition diagrams for the binary mixtures DME + SO(2) at 363.15 K and DME + CO(2) at 335.15 and 308.15 K. For the DME + SO(2) mixture, simulation is able to qualitatively reproduce the minimum pressure azeotropy observed experimentally for this mixture, but quantitative errors exist, suggesting that multibody effects may be important in this system. For the DME + CO(2) mixture, simulation is able to predict the pressure-composition behavior within 1% of experimental data. Simulations in the isobaric-isothermal ensemble are used to determine the microstructure of DME + SO(2) and DME + CO(2) mixtures. The DME + SO(2) shows weak pairing between DME and SO(2) molecules, while no specific pairing or aggregation is observed for mixtures of DME + CO(2).

Phase behavior of a two-dimensional and confined in slitlike pores square-shoulder, square-well fluid

We report the phase diagrams for a square-shoulder, square-well fluids in two dimensions (2D), as well as confined in slitlike pores. The diagrams are obtained by histogram reweighting Monte Carlo simulations. The critical points are located by using finite size scaling analysis. Our calculations indicate that the phase behavior of the systems investigated is distinct from that observed in three dimensions. In particular, we have not found the liquid-liquid transition that occurs in the bulk.

Mie Potentials for Phase Equilibria Calculations: Application to Alkanes and Perfluoroalkanes

Transferable united-atom force fields, based on n - 6 Lennard-Jones potentials, are presented for normal alkanes and perfluorocarbons. It is shown that by varying the repulsive exponent the range of the potential can be altered, leading to improved predictions of vapor pressures while also reproducing saturated liquid densities to high accuracy. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the vapor liquid coexistence curves, vapor pressures, heats of vaporization, and critical points for normal alkanes methane through tetradecane, and perfluorocarbons perfluoromethane through perfluorooctane. For all molecules studied, saturated liquid densities are reproduced to within 1% of experiment. Vapor pressures for normal alkanes and perfluorocarbons were predicted to within 3% and 6% of experiment, respectively. Calculations performed for binary mixture vapor-liquid equilibria for propane + pentane show excellent agreement with experiment, while slight deviations are observed for the ethane + perfluoroethane mixture.

Low-temperature fluid-phase behavior of ST2 water

We perform histogram-reweighting Monte Carlo simulations of the ST2 model of water in the grand-canonical ensemble in order to investigate its low-temperature fluid-phase behavior. Using Ewald summation treatment of long-range electrostatic interactions, we locate the critical point of the liquid-liquid transition at T=237±4 K, ρ=0.99±0.02 g/cc, P=167±24 MPa. Contrary to previous reports in the literature [Brovchenko et al., J. Chem. Phys. 118, 9473 (2003); Brovchenko et al., J. Chem. Phys. 123, 044515 (2005)], according to which there are three liquid-liquid transitions in ST2 with simple truncation of electrostatic interactions, and two in ST2 with reaction field treatment of long-range Coulombic forces, we find only one liquid-liquid transition. Our work points to the sensitivity of results to the proper treatment of electrostatic interactions, and to the introduction of artificial constraints that limit the magnitude of density fluctuations.

Extension of the Transferable Potentials for Phase Equilibria Force Field to Dimethylmethyl Phosphonate, Sarin, and Soman

The transferable potentials for phase equilibria force field is extended to dimethylmethylphosphonate (DMMP), sarin, and soman by introducing a new interaction site representing the phosphorus atom. Parameters for the phosphorus atom are optimized to reproduce the liquid densities at 303 and 373 K and the normal boiling point of DMMP. Calculations for sarin and soman are performed in predictive mode, without further parameter optimization. Vapor-liquid coexistence curves, critical properties, vapor pressures and heats of vaporization are predicted over a wide range of temperatures with histogram reweighting Monte Carlo simulations in the grand canonical ensemble. Excellent agreement with experiment is achieved for all compounds, with unsigned errors of less than 1% for vapor pressures and normal boiling points and under 5% for heats of vaporization and liquid densities at ambient conditions.

Effect of torsional potential on the predicted phase behavior of n-alkanes

The effect of torsional potential on the predictions of simulation for vapor–liquid equilibria of n-alkanes is determined. Calculations are performed with histogram-reweighting Monte Carlo simulations in the grand canonical ensemble. Decreasing the magnitude of energy barriers to dihedral rotation or allowing free rotation is found to have no effect on the predicted vapor–liquid equilibria. Restriction of the dihedral angles to a Gaussian distribution around the minimum energy conformation causes an under-prediction of the liquid densities and critical temperatures by a maximum of 7% and 2%, respectively, with discrepancies increasing monotonically with the number of dihedral angles present in a molecule. No significant deviation in vapor pressure is observed for any compound, regardless of torsional potential used. An analysis of the conformational behavior reveals that restriction of the dihedral sampling has a measurable effect on excluded volume of the molecule, and this change of conformational behavior is responsible for the reduction in the predicted saturated liquid densities observed in this work. Similar calculations for force fields employing reduced dihedral potentials or freely jointed chains show little change in the predicted excluded volume compared to the reference force field.

Phase diagram of a square-shoulder, square-well fluid revisited

We report here the phase diagrams for a model square-shoulder, square-well fluid obtained by histogram reweighting Monte Carlo simulations. The critical points corresponding to the gas-liquid condensation and to the liquid-liquid transition have been located. Also, we have estimated the temperatures and densities at the triple points. We believe that our results are more precise and complete than those published so far.

Development of the TraPPE-UA force field for ethylene oxide

A united-atom force field for ethylene oxide (oxirane) is presented. Parameters for both the dispersive and electrostatic interactions are simultaneously determined by fitting to the experimentally determined critical point, the normal boiling point and two vapor pressures. Vapor and liquid coexistence densities, heats of vaporization and vapor pressures are determined with histogram-reweighting Monte Carlo simulations in the grand canonical ensemble. NpT simulations are used to determine the heat capacity and isothermal compressibility.

Vapor−Liquid Equilibria of Diethylamine + Methanol, Diethylamine + Acetone and Diethylamine + Acetonitrile: Predictions of Atomistic Computer Simulations

Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine pressure−composition diagrams for diethylamine + methanol, diethylamine + acetone and diethylamine + acetonitrile. Simulations were unable to predict the polyazeotropy found in the diethylamine + methanol mixture at 398 K or the minimum pressure azeotropy found at 348 and 298 K. Simulation predictions of the pressure−composition behavior of diethylamine + acetone and diethylamine + acetonitrile, however, were found to be in close agreement with experiment. Simulations in the isobaric−isothermal ensemble were used to determine the structure of the liquid phase for each of the mixtures described previously and revealed significant association taking place between unlike species.

Effect of partial charge parametrization on the fluid phase behavior of hydrogen sulfide

The effect of partial charge parametrization on the fluid phase behavior of hydrogen sulfide is investigated with grand canonical histogram reweighting Monte Carlo simulations. Four potential models, based on a Lennard-Jones + point charge functional form, are developed. It is shown that Lennard-Jones parameters can be tuned such that partial charges for the sulfur atom in the range -0.40 < q(s) < -0.252 will lead to an accurate reproduction of experimental vapor-liquid equilibria. Each of the parameter sets developed in this work are used to predict the pressure composition behavior H2S-n-pentane at 377.6 K. While the mixture calculation provides a means of reducing the number of candidate parameter sets, multiple parameter sets were found to yield an excellent reproduction of both the pure component and mixture phase behavior.

Molecular Modeling of Phase Behavior and Microstructure of Acetone−Chloroform−Methanol Binary Mixtures

Force fields based on a Lennard-Jones (LJ) 12-6 plus point charge functional form are developed for acetone and chloroform specifically to reproduce the minimum pressure azeotropy found experimentally in this system. Point charges are determined from a CHELPG population analysis performed on an acetone-chloroform dimer. The required electrostatic surface for this dimer is determined from ab initio calculations performed with MP2 theory and the 6-31g++(3df,3pd) basis set. LJ parameters are then optimized such that the liquid-vapor coexistence curve, critical parameters, and vapor pressures are well reproduced by simulation. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the phase diagrams for the binary mixtures acetone-chloroform, acetone-methanol, and chloroform-methanol. The force fields developed in this work reproduce the minimum pressure azeotrope in the acetone-chloroform mixture found in experiment. The predicted azeotropic composition of x(CHCl3) = 0.77 is in fair agreement with the experimental value of x(CHCl3)expt = 0.64. The new force fields were also found to provide improved predictions of the pressure-composition behavior of acetone-methanol and chloroform-methanol when compared to other force fields commonly used for vapor-liquid equilibria calculations. NPT simulations were conducted at 300 K and 1 bar for equimolar mixtures of acetone-chloroform, acetone-methanol, and methanol-chloroform. Analysis of the microstructure reveals significant hydrogen bonding occurring between acetone and chloroform. Limited interspecies hydrogen bonding was found in the acetone-methanol or chloroform-methanol mixtures.

Effect of quadrupole moment on the phase behavior of binary mixtures containing ethene

Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the effect of quadrupole moment on the phase behavior of binary mixtures containing ethene. Two united-atom force fields for ethene are developed, one with an explicit representation of the quadrupole moment via partial charges, the other without. Simulations demonstrate that the addition of an explicit quadrupole moment results in a slight improvement in the reproduction of pure ethene vapor–liquid equilibria. For mixtures of xenon/ethene, the predictions of simulation are also improved by the use of an explicit quadrupole moment. For CO 2/ethane mixtures, however, the quadrupole explicit force field fails to predict the maximum pressure azeotrope found experimentally, while the force field without a quadrupole moment is in quantitative agreement with experiment. For n-butane/ethene mixtures the quadrupole moment has little effect on the predicted pressure composition behavior.

An Improved Force Field for the Prediction of the Vapor−Liquid Equilibria for Carboxylic Acids

An improved united-atom force field, based on a Lennard-Jones plus point charge functional form, for carboxylic acids is proposed. Point charges are determined from a Mulliken population analysis of ab initio calculations performed at the MP2 level of theory with the 6-31+g(d,p) basis set. The Lennard-Jones well depth and size parameters for the carboxyl carbon interaction site were determined by fitting to single-component vapor−liquid equilibrium data for acetic acid, while the remaining Lennard-Jones parameters were taken from the TraPPE-UA force field. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble were used to determine the vapor−liquid coexistence curves, vapor pressures, and critical points predicted by the new force field. The new force field was found to describe accurately the phase equilibria of acetic acid, with mean unsigned errors in the saturated liquid density of less than 1.6%. The critical temperature and density predicted by the new force field were within...

Grand canonical Monte Carlo simulations of phase equilibria of pure silicon tetrachloride and its binary mixture with carbon dioxide

Grand canonical histogram-reweighting Monte Carlo simulations were used to obtain the phase behaviour of pure silicon tetrachloride and its binary mixture with carbon dioxide. Two new potential models for pure silicon tetrachloride were developed and parametrized to the vapour-liquid coexistence properties. The first model, with one exponential-6 site and fixed electrostatic charges on atoms, does not adequately reproduce the experimental phase behaviour due to its inability to represent orientational anisotropy in the liquid phase. The second potential model, with five exponential-6 sites for the repulsive and dispersive interactions plus partial charges, accurately reproduces experimental saturated liquid and vapour densities as well as vapour pressures and the second virial coefficient for pure silicon tetrachloride. This model was used in simulations of the phase behaviour of the binary mixture carbon dioxide-silicon tetrachloride. Two sets of combining rules (Lorentz-Berthelot and Kong [1973, J. chem. Phys., 59, 2464]) were used to obtain unlike-pair potential parameters. For the binary system, the predicted phase diagram is in good agreement with experiment when the Kong combining rules are used. The Lorentz-Berthelot rules significantly overestimate the solubility of carbon dioxide in silicon tetrachloride.

Vapor−Liquid Phase Equilibria for Linear and Branched Alkane Monolayers Physisorbed on Au(111)

Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble were used to obtain vapor−liquid coexistence curves for a series of alkanes physisorbed on a flat gold substrate. The critical temperatures and densities of n-alkanes from methane to decane as well as the branched molecules 2-methylpropane, 2,2-dimethylbutane, and 2,3-dimethylbutane were determined through a mixed-field analysis. The ratio of the 2D (two-dimensional) to the 3D (three-dimensional) critical temperature was found to depend weakly on the chain length for n-alkanes, decreasing from Tc2D/Tc3D = 0.38 for methane to Tc2D/Tc3D = 0.31 for n-decane. In contrast to typical bulk fluid behavior, the branched isomers were found to have higher critical temperatures than their linear counterparts.

The hydrophobic hydration of methane as a function of temperature from histogram reweighting Monte Carlo simulations

A microscopic explanation for the experimental observations of the solvation of methane in water is proposed by way of free energy and structural information estimated using computer simulations. In particular, the temperature dependence of both the free energy of solvation as well as structural information is calculated. The free energy is estimated by reweighting histograms from canonical and grand canonical Monte Carlo simulations along a thermodynamic path. The results for the free energy and structural changes are found to be in qualitative agreement with experimental data. We propose that the decrease in entropy found in the solvation process is brought about by excluded volume effects on mixing the two components rather than any local “freezing” of the water around the methane. At the same time the enthalpy of solvation is determined by the solute-solvent interaction. A weak dependence of the hydration free energy with temperature is found. A possible explanation lies with the relatively low temperature sensitivity of the methane–water radial distribution functions.