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.
5An optimized molecular model for ammonia, which is based on a previ-6 ous work of Kristóf et al., Mol. Phys. 97 (1999) 1129-1137 Improvements are achieved by including data on geometry and electro-8 statics from ab initio quantum mechanical calculations in a first model. 9 Afterwards the parameters of the Lennard-Jones potential, modeling dis-10 persive and repulsive interactions, are optimized to experimental vapor-11 liquid equilibrium data of pure ammonia. The resulting molecular model 12 shows mean unsigned deviations to experiment of 0.7 % in saturated liq-13 uid density, 1.6 % in vapor pressure, and 2.7 % in enthalpy of vaporization 14 over the whole temperature range from triple point to critical point. This 15 new molecular model is used to predict thermophysical properties in the 16 liquid, vapor and supercritical region, which are in excellent agreement 1 Introduction 24Molecular modeling and simulation is a powerful tool for predicting thermo-25 physical properties, that is becoming more accesible due to the ever increasing 26 computing power and the progress of methods and simulation tools. For real 27 life applications in process engineering reliable predictions are needed for a wide 28 variety of properties [1, 2, 3]. 29 The central role for that task is played by the molecular model, that de-30 termines all of them. Therefore, a balanced modeling procedure, i.e. selection 31 of model type and parameterization, is crucial. Unfortunately, thermophysi-32 cal properties usually depend on the model parameters in a highly non-linear 33 fashion. So the development of new molecular models of technical quality is a 34 time-consuming task. In this paper a procedure is proposed that uses informa-35 tion from ab initio quantum mechanical calculations to accelerate the modeling 36 process. As an example, ammonia is regarded here. 37 Ammonia is a well-known chemical intermediate, mostly used in fertilizer 38 industries; another important application is its use as a refrigerant. Due to its 39 simple symmetric structure and its strong intermolecular interactions it is also 40 of high academic interest both experimentally and theoretically. 41 Different approaches can be found in the literature to construct an inter-42 molecular potential for ammonia to be used in molecular simulation. Jorgensen 43 and Ibrahim [4] as well as Hinchliffe et al. [5] used experimental bond distances 44 and angles to place their interaction sites. Jorgensen and Ibrahim fitted a 12-6-3 45 potential plus four partial charges to results from ab initio quantum mechanical 46 calculations, they derived for 250 orientations of the ammonia dimer using the 47 STO-3G minimal basis set. To yield reasonable potential energies for liquid 48 ammonia compared to experimental results, they had to scale their potential by 49 a factor 1.26. 50 Hinchliffe et al. used a combination of exponential repulsion terms, an at-51 tractive Morse potential, and four partial charges to construct the intermolecular 52 2 potential. The par...
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