Bernhard Eckl

Set of Molecular Models Based on Quantum Mechanical Ab Initio Calculations and Thermodynamic Data

A parametrization strategy for molecular models on the basis of force fields is proposed, which allows a rapid development of models for small molecules by using results from quantum mechanical (QM) ab initio calculations and thermodynamic data. The geometry of the molecular models is specified according to the atom positions determined by QM energy minimization. The electrostatic interactions are modeled by reducing the electron density distribution to point dipoles and point quadrupoles located in the center of mass of the molecules. Dispersive and repulsive interactions are described by Lennard-Jones sites, for which the parameters are iteratively optimized to experimental vapor-liquid equilibrium (VLE) data, i.e., vapor pressure, saturated liquid density, and enthalpy of vaporization of the considered substance. The proposed modeling strategy was applied to a sample set of ten molecules from different substance classes. New molecular models are presented for iso-butane, cyclohexane, formaldehyde, dimethyl ether, sulfur dioxide, dimethyl sulfide, thiophene, hydrogen cyanide, acetonitrile, and nitromethane. Most of the models are able to describe the experimental VLE data with deviations of a few percent.

An optimised molecular model for ammonia

An optimised molecular model for ammonia, which is based on a previous work of Kristóf et al. [Mol. Phys. 97, 1129 (1999)], is presented. Improvements are achieved by including data on geometry and electrostatics from ab initio quantum mechanical calculations in a first model. Afterwards the parameters of the Lennard–Jones potential, modelling dispersive and repulsive interactions, are optimised to experimental vapour–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 vapour pressure, and 2.7% in enthalpy of vapourisation over the whole temperature range from triple point to critical point. This new molecular model is used to predict thermophysical properties in the liquid, vapour and supercritical region, which are in excellent agreement with a high precision equation of state, that was optimised to 1147 experimental data sets. Furthermore, it is also capable of predicting the radial distribution functions properly, while no structural information is used in the optimisation procedure.

Molecular Modeling and Simulation of Thermophysical Properties: Application to Pure Substances and Mixtures

At the Institute for Thermodynamics and Thermal Process Engineering (ITT) about 100 molecular models for pure substances have been developed so far. These models reproduce vapor pressure, saturated liquid density, and enthalpy of vaporization with a technical accuracy of less than 5%, less than 1%, and less than 5% deviation compared to experimental data, respectively. Simulation results are outlined for the pure substances ethylene oxide and ammonia, demonstrating the outstanding predictive power of the applied methods. Secondly, nucleation processes are discussed. Underlying effects on are studied on the molecular level.

Molecular Modeling of Hydrogen Bonding Fluids: Formic Acid and Ethanol + R227ea

Currently, molecular modeling and simulation gains importance for the prediction of thermophysical properties of pure fluids and mixtures, both in research and industry. This is due to several reasons: Firstly, the predictive power of molecular models allows for results with technically interesting accuracies over wide range of state points and makes it superior to classical methods. Secondly, a given molecular model provides access to the full variety of thermophysical properties, such as thermal, caloric, transport or phase equilibrium data. Finally, through the advent of cheaply available powerful computing infrastructure, reasonable execution times for molecular simulations can be achieved. Molecular modeling and simulation are based on statistical thermodynamics which links the intermolecular interactions to the macroscopic thermophysical properties. This sound physical background also supports the increasing acceptance compared to phenomenological modeling.