A new version release (4.0) of the molecular simulation tool ms2 (Deublein et al., 2011; is presented. Version 4.0 of ms2 features two additional potential functions to address the repulsive and dispersive interactions in a more versatile way, i.e. the Mie potential and the Tang-Toennies potential. This version further introduces Kirkwood-Buff integrals based on radial distribution functions, which allow the sampling of the thermodynamic factor of mixtures with up to four components, orientational distribution functions to elucidate mutual configurations of neighboring molecules, thermal diffusion coefficients of binary mixtures for heat, mass as well as coupled heat and mass transport, Einstein relations to sample transport properties with an alternative to the Green-Kubo formalism, dielectric constant of non-polarizable fluid models, vapor-liquid equilibria relying on the second virial coefficient and cluster criteria to identify nucleation.
Vapor-liquid equilibria and fluid interface properties of binary mixtures containing either methane or cyclohexane representing fuel and nitrogen or oxygen are reported. The mixtures are studied at different temperatures and pressures, which are chosen such that the temperature of the fuel component is subcritical, while that of the gaseous component is mainly supercritical. Data are obtained from molecular dynamics (MD) simulation, as well as density functional theory and density gradient theory in combination with the PC-SAFT equation of state (EOS). The studied interface properties include surface tension, interface thickness, enrichment and relative adsorption. Furthermore, bulk vapor-liquid equilibrium properties are computed with two distinct MD methods as well as the PC-SAFT EOS. All approaches are compared to data from highly accurate empirical EOS. Despite the fundamental differences between these methods, very good agreement between the results of MD, density functional theory as well as density gradient theory and EOS data is observed for the phase equilibria and interface properties, reinforcing the present predictions. It is found that the equivalence of nitrogen to oxygen is rather limited, in particular for the methane propellant. The disparities are primarily pronounced for low temperatures, where the compositions of the bulk phases differ significantly. As a result, enthalpy of vaporization as well as surface tension attain much higher values for mixtures containing oxygen.
Generalized expressions for thermodynamic properties
in terms of
ensemble averages are discussed for adiabatic and isothermal ensembles.
They are implemented in the simulation code ms2 and
validated by Monte Carlo simulations for the Lennard-Jones fluid.
A comparison of the eight statistical ensembles regarding size scaling
behavior, convergence, and stability is provided for state points
throughout the homogeneous fluid region. The resulting data are in
good agreement but differ in their statistical distributions. In closed
systems, the statistical quality of the data is better than in open
systems. Overall, the microcanonical ensemble performs best.
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