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We present an approach to incorporate the effect of vibrational nonequilibrium in molecular dynamics (MD) simulations. A perturbed canonical ensemble, in which selected modes are excited to higher temperature while all others remain equilibrated at low temperature, is simulated by applying a specifically tailored bias potential. Our method can be readily applied to any (classical or quantum mechanical) MD setup at virtually no additional computational cost and allows the study of reactions of vibrationally excited molecules in nonequilibrium environments such as plasmas. In combination with enhanced sampling methods, the vibrational efficacy and mode selectivity of vibrationally stimulated reactions can then be quantified in terms of chemically relevant observables, such as reaction rates and apparent free energy barriers. We first validate our method for the prototypical hydrogen exchange reaction and then show how it can capture the effect of vibrational excitation on a symmetric SN2 reaction and radical addition on CO2.

Azeotropes have been studied for decades due to the challenges they impose on separation processes but fundamental understanding at the molecular level remains limited. Although molecular simulation has demonstrated its capability of predicting mixture vapor–liquid equilibrium (VLE) behaviors, including azeotropes, its potential for mechanistic investigation has not been fully exploited. In this study, we use the united atom transferable potentials for phase equilibria (TraPPE-UA) force field to model the ethanol/benzene mixture, which displays a positive azeotrope. Gibbs ensemble Monte Carlo (GEMC) simulation is performed to predict the VLE phase diagram, including an azeotrope point. The results accurately agree with experimental measurements. We argue that the molecular mechanism of azeotrope formation cannot be fully understood by studying the mixture liquid-state stability at the azeotrope point alone. Rather, azeotrope occurrence is only a reflection of the changing relative volatility between the two components over a much wider composition range. A thermodynamic criterion is thus proposed on the basis of the comparison of partial excess Gibbs energy between the components. In the ethanol/benzene system, molecular energetics shows that with increasing ethanol mole fraction, its volatility initially decreases but later plateaus, while benzene volatility is initially nearly constant and only starts to decrease when its mole fraction is low. Analysis of the mixture liquid structure, including a detailed investigation of ethanol hydrogen-bonding configurations at different composition levels, reveals the underlying molecular mechanism for the changing volatilities responsible for the azeotrope. Natural Sciences and Engineering Research Council (NSERC) of Canada (RGPIN-4903-2014); National Natural Science Foundation of China (NFSC; No. 21878219); China Scholarship Council (CSC; No. 201500090106).

In this work, we present improved estimates of the critical temperature (Tc), critical density (ρc), critical pressure (Pc), and critical compressibility factor (Zc) for n-alkanes with chain lengths as large as C48. These are obtained for several different force field models with Gibbs ensemble Monte Carlo simulations. We implement a recently proposed data analysis method designed to reduce the uncertainty in Tc, ρc, Pc, and Zc when predicted with molecular simulation. The results show a large reduction in the uncertainties compared to the simulation literature with the greatest reduction found for ρc, Pc, and Zc. Previously, even the most computationally intensive molecular simulation studies have not been able to elucidate the n-alkane Pc trend with respect to larger carbon numbers. The results of this study are significant because the uncertainty in Pc is small enough to discern between conflicting experimental data sets and prediction models for large n-alkanes. Furthermore, the results for Tc resolve...

The shear viscosities of n-decane and 4-propylheptane have been calculated over a range of densities. Equilibrium molecular dynamics simulations were carried out in the canonical ensemble for an empirical force field using the united-atom representation. Quantitative agreement is obtained with the experimental shear viscosities and the pressure−viscosity coefficient.

The separation performance of the metal−organic framework MIL-47 for xylene isomers was studied by Monte Carlo simulations in the grand-canonical ensemble. The experimental adsorption isotherms of xylene isomers in MIL-47 between 343 and 423 K were reproduced by using force fields available in the literature. Mixture isotherms were computed and compared with the mixture isotherms predicted by using pure component adsorption data and the ideal adsorption solution theory. This theory accurately predicts mixture isotherms of xylene isomers in MIL-47. the ideal adsorption sorption theory was further employed to calculate the separation factors of xylene isomers in MIL-47. The order of preferential adsorption was found to be ortho > para > meta. The adsorption selectivity was found to increase with pressure, and the results showed a good agreement with the experimental data available. Henry coefficients and low coverage heats of adsorption and adsorption entropies were computed at 543 K showing an excellent ag...

We present canonical-ensemble molecular-dynamics (MD) simulations of disjoining-pressure isotherms for symmetric, free aqueous thin films. For such symmetric films, the disjoining pressure is purely attractive. Lifshitz's theory, based on continuum dispersion forces, predicts that the disjoining pressure varies as an inverse cube of film thickness, with a constant of proportionality that can be calculated within the framework of this theory. Our MD results indicate that Lifshitz theory, which assumes a slab geometry for the water density profile and neglects the fluid structure, underpredicts the disjoining pressure by about 50 times for films ranging from about 1 to 2 nm at 479 K. To investigate more closely actual experimental conditions, we also perform simulations of water films surrounded by inert gas molecules. The additional gas component adds an extra thermodynamic degree of freedom to the system, allowing for the chemical potential of the water in the external liquid reservoir to be maintained co...