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By introducing a common representational system for metadata that describe the employed simulation workflows, diverse sources of data and platforms in computational molecular engineering, such as workflow management systems, can become interoperable at the semantic level. To achieve semantic interoperability, the present work introduces two ontologies that provide a formal specification of the entities occurring in a simulation workflow and the relations between them: The software ontology VISO is developed to represent software packages and their features, and OSMO, an ontology for simulation, modelling, and optimization, is introduced on the basis of MODA, a previously developed semi-intuitive graph notation for workflows in materials modelling. As a proof of concept, OSMO is employed to describe a use case of the TaLPas workflow management system, a scheduler and workflow optimizer for particle-based simulations.

We study a rectifying mutant of the OmpF porin ion channel using both all-atom and reduced models. The mutant was created by Miedema et al. [Nano Lett., 2007, 7, 2886] on the basis of the N-P semiconductor diode, in which an N-P junction is formed. The mutant contains a pore region with positive amino acids on the left-hand side and negative amino acids on the right-hand side. Experiments show that this mutant rectifies. Although we do not know the structure of this mutant, we can build an all-atom model for it on the basis of the structure of the wild type channel. Interestingly, molecular dynamics simulations for this all-atom model do not produce rectification. A reduced model that contains only the important degrees of freedom (the positive and negative amino acids and free ions in an implicit solvent), on the other hand, exhibits rectification. Our calculations for the reduced model (using the Nernst-Planck equation coupled to Local Equilibrium Monte Carlo simulations) reveal a rectification mechanism that is different from that seen for semiconductor diodes. The basic reason is that the ions are different in nature from electrons and holes (they do not recombine). We provide explanations for the failure of the all-atom model including the effect of all the other atoms in the system as a noise that inhibits the response of ions (that would be necessary for rectification) to the polarizing external field., Comment: 16 pages, 10 figures

A set of molecular models for 78 pure substances from prior work is taken as a basis for systematically studying vapor-liquid equilibria (VLE) in ternary systems.All 33 ternary mixtures of these 78 components for which experimental VLE data is available are studied by molecular simulation. The mixture models are based on the modified Lorentz-Berthelot combining rule that contains one binary interaction parameter which was adjusted to a single experimental binary vapor pressure of each binary subsystem in prior work.No adjustment to ternary data is carried out. The predictions from the molecular models of the 33 ternary mixtures are compared to the available experimental data.In almost all cases, the molecular models give excellent predictions of the ternary mixture properties., Comment: Fluid Phase Equilibria, submitted (2009)

Equilibrium molecular dynamics simulation and the Green-Kubo formalism were used to calculate self-diffusion coefficient, shear viscosity,and thermal conductivity for 30 different quadrupolar two-center Lennard-Jones fluids along the bubble line and in the homogeneous liquid. It was systematically investigated how anisotropy, i.e. elongation, and quadrupole momentum influence the transport properties. The reduced elongation L* was varied from 0 to 0.8 and the reduced squared quadrupole momentum Q*2 from 0 to 4, i.e. in the entire range in which parameters for real fluids are expected. The statistical uncertainty of the reported data varies with transport property, for self-diffusion coefficient data the error bars are typically lower than 3 %, for shear viscosity and thermal conductivity they are about 8 and 12 %, respectively.

Equilibrium molecular dynamics simulation and the Green-Kubo formalism were used to calculate self-diffusion coefficient, shear viscosity, and thermal conductivity for 38 different dipolar two-center Lennard-Jones fluids along the bubble line and in the homogeneous liquid. It was systematically investigated how anisotropy, i.e. elongation, and dipole momentum influence the transport properties. The reduced elongation L* was varied from 0 to 1 and the reduced squared dipole momentum was varied depending on the elongation as follows: for spherical fluids (L*=0) from $\mu*2=0 to 20, for L*=0.2 from mu*2=0 to 16, and for L*=0.4 and above from mu*2=0 to 12. This represents the entire range in which parameters for real fluids are expected. The statistical uncertainty of the reported data varies with transport property, for self-diffusion coefficient data the error bars are typically lower than 3 %, for shear viscosity and thermal conductivity they are about 8 and 12 %, respectively.

Shear and bulk viscosity and thermal conductivity for argon, krypton, xenon, and methane and the binary mixtures argon+krypton and argon+methane were determined by equilibrium molecular dynamics with the Green-Kubo method. The fluids were modeled by spherical Lennard-Jones pair-potentials with parameters adjusted to experimental vapor liquid-equilibria data alone. Good agreement between the predictions from simulation and experimental data is found for shear viscosity and thermal conductivity of the pure fluids and binary mixtures. The simulation results for the bulk viscosity show only poor agreement with experimental data for most fluids, despite good agreement with other simulations data from the literature. This indicates that presently available experimental data for the bulk viscosity, a property which is difficult to measure, are inaccurate.

A new molecular simulation method for the calculation of vapour-liquid equilibria of mixtures is presented. In this method, the independent thermodynamic variables are temperature and liquid composition. In the first step, one isobaric isothermal simulation for the liquid phase is performed, in which the chemical potentials of all components and their derivatives with respect to the pressure, i.e., the partial molar volumes, are calculated. From these results, first order Taylor series expansions for the chemical potentials as functions of the pressure $\mu_i(p)$ at constant liquid composition are determined. That information is needed, as the specified pressure in the liquid will generally not be equal to the equilibrium pressure, which has to be found in the course of a vapour simulation. In the second step, one pseudo grand canonical simulation for the vapour phase is performed, where the chemical potentials are set according to the instantaneous pressure $p^v$ using the previously determined function $\mu_i(p^v)$. In this way, results for the vapour pressure and vapour composition are achieved which are consistent to the given temperature and liquid composition.The new method is applied to the pure Lennard-Jones fluid, a binary, and a ternary mixture of Lennard-Jones spheres and shows very good agreement with corresponding data from the literature.

noindent Henry's law constants of the solutes methane, nitrogen, oxygen and carbon dioxide in the solvent ethanol are predicted by molecular simulation. The molecular models for the solutes are taken from previous work. For the solvent ethanol, a new rigid anisotropic united atom molecular model based on Lennard-Jones and Coulombic interactions is developed. It is adjusted to experimental pure component saturated liquid density and vapor pressure data. Henry's law constants are calculated by evaluating the infinite dilution residual chemical potentials of the solutes from 273 to 498K with Widom's test particle insertion. The prediction of Henry's Law constants without the use of binary experimental data on the basis of the Lorentz-Berthelot combining rule agree well with experimental data, deviations are 20%, except for carbon dioxide for which deviations of 70% are reached. Quantitative agreement is achieved by using the modified Lorentz-Berthelot combining rule which is adjusted to one experimental mixture data point.

The gradual insertion method for direct calculation of the chemical potential by molecular simulation is applied in the NpT ensemble to different quadrupolar two-centre Lennard-Jones fluids at high density state points. The results agree well with Widom's test particle insertion but show at very high densities significantly smaller statistical uncertainties. The gradual insertion method, which is coupled here with preferential sampling, extends the density range where reliable information on the chemical potential can be obtained. Application details are reported.

For the design and optimization of CO2 recovery from alcoholic fermentation processes by distillation, models for vapor-liquid equilibria (VLE) are needed. Two such thermodynamic models, the Peng-Robinson equation of state (EOS) and a model based on Henry's law constants, are proposed for the ternary mixture N2+O2+CO2. Pure substance parameters of the Peng-Robinson EOS are taken from the literature, whereas the binary parameters of the Van der Waals one-fluid mixing rule are adjusted to experimental binary VLE data. The Peng-Robinson EOS describes both binary and ternary experimental data well, except at high pressures approaching the critical region. A molecular model is validated by simulation using binary and ternary experimental VLE data. On the basis of this model, the Henry's law constants of N2 and O2 in CO2 are predicted by molecular simulation. An easy-to-use thermodynamic model, based on those Henry's law constants, is developed to reliably describe the VLE in the CO2-rich region.

An optimized molecular model for ammonia, which is based on a previous work of Kristoef et al., Mol. Phys. 97 (1999) 1129--1137, is presented. Improvements are achieved by including data on geometry and electrostatics from quantum mechanical calculations in a first model. Afterwards the parameters of the Lennard-Jones potential, modeling dispersive and repulsive interactions, are optimized to experimental vapor-liquid equilibrium data of pure ammonia. The resulting molecular model shows mean unsigned deviations to experiment of 0.7% in saturated liquid density, 1.6% in vapor pressure, and 2.7% in enthalpy of vaporization over the whole temperature range from triple point to critical point. This new molecular model is used to predict thermophysical properties in the liquid, vapor and supercritical region, which are in excellent agreement with a high precision equation of state, that was optimized to 1147 experimental data sets. Furthermore, it is also capable to predict the radial distribution functions properly, while no structural information is used in the optimization procedure.

Self diffusion coefficients and binary Maxwell-Stefan diffusion coefficients were determined by equilibrium molecular dynamics simulations with the Green-Kubo method. The study covers five pure fluids: neon, argon, krypton, xenon, and methane and three binary mixtures: argon+krypton, argon+xenon, and krypton+xenon. The fluids are modeled by spherical Lennard-Jones air-potentials, with parameters which were determined solely on the basis of vapor-liquid equilibria data. The predictions of the self diffusion coefficients agree within 5% for gas state points and about 10% for liquid state points. The Maxwell-Stefan diffusion coefficients are predicted within 10%. A test of Darken's model shows good agreement.

A molecular model for carbon dioxide is assessed regarding vapor-liquid quilibrium properties.Large deviations, being above 15 %, are found for vapor pressure and saturated vapor density in the entire temperature range.

The influence of the unlike Lennard-Jones (LJ) parameters on vapor-liquid equilibria of mixtures is investigated and the performance of eleven combining rules is assessed. In the first part of the work, the influence of the unlike LJ size and energy parameter on vapor pressure, bubble density and dew point composition is systematically studied for the mixtures CO+$\rm C_2H_6$ and $\rm N_2$ + $\rm C_3H_6$, respectively. It is found that mixture vapor pressure depends strongly both on the size and the energy parameter whereas the bubble density depends mostly on the size parameter and the dew point composition is rather insensitive to both parameters. In preceding work, unlike LJ parameters were adjusted to experimental binary vapor-liquid equilibria for 44 real mixtures. On the basis of these results, in the second part of the work eleven combining rules are assessed regarding their predictive power. A comparison with the adjusted unlike LJ parameters determined from the fit shows that none of the eleven combining rules yields appropriate parameters in general. To obtain an accurate mixture model, the unlike dispersive interaction should therefore be adjusted to experimental binary data. The results from the present work indicate that it is sufficient to use the Lorenz rule for the unlike LJ size parameter and to fit the unlike LJ energy parameter to the vapor pressure.

A molecular model for carbon dioxide is assessed regarding vapor-liquid equilibrium properties. Large deviations, being above 15 %, are found for vapor pressure and saturated vapor density in the entire temperature range.

Results of a systematic investigation of the vapour-liquid equilibria (VLE) of 30 individual two-centre Lennard-Jones plus pointquadrupole model fluids (2CLJQ) are reported over a range of reduced quadrupolar momentum $0 \le Q^{*2} \le 4$ and of reduced elongation $0 \le L^* \le 0.8$. Temperatures investigated are from about 55 % to about 95 % of the critical temperature of each fluid. Uniformly the $NpT$+Test Particle Method based on molecular dynamics simulations is used for the generation of vapour pressures, saturated densities, and saturated enthalpies. Critical temperatures $T^*_{\rm c}$ and densities $\rho^*_{\rm c}$ are obtained from Guggenheim's equations. Empirical correlations for critical data $T^*_{\rm c}$ and $\rho^*_{\rm c}$ as well as for saturated densities $\rho'^*$, $\rho''^*$, and vapour pressures $p^*_{\sigma}$ are developed as global functions of the model parameters. In most cases they describe the simulation data within their statistical uncertainties. Critical pressures and acentric factors of the 2CLJQ fluid can be calculated from present correlations. The present simulations are a sound basis for adjustments of the model parameters $Q^{*2}$, $L^*$, $\sigma$, and $\epsilon$ to experimental VLE data of real fluids.

Molecular modelling and simulation as well as four equations of state (EOS) are applied to natural gas mixtures regarding Joule-Thomson (JT) inversion. JT inversion curves are determined by molecular simulation for six different natural gas mixtures consisting of methane, nitrogen, carbon dioxide and ethane. These components are also regarded as pure fluids, leading to a total of ten studied systems. The results are compared to four advanced mixture EOS: DDMIX, SUPERTRAPP, BACKONE and the recent GERG-2004 Wide-Range Reference EOS. It is found that molecular simulation is competitive with state-of-the-art EOS in predicting JT inversion curves. The molecular based approaches (simulation and BACKONE) are superior to DDMIX and SUERTRAPP.