(469e) Development of Force Fields for Molecular Simulations Including Phase and Transport Properties

Force fields for molecular simulations are usually developed based on phase equilibria properties. However, it has been proved that most of them fail when applied to estimate properties different to those from which they have been parameterized, especially transport properties. This work deals with the development of force fields by simultaneous fitting all force field parameters, using as reference data both transport properties (shear viscosity) and several equilibrium properties (vapor pressure, saturated liquid density and surface tension). Force field parameters have been adjusted by a multivariable optimization procedure very similar to the one proposed by Ungerer and co-workers [1]. We will present here the application of the methodology to SF6 [2]. The force field contains two terms, dealing with the intermolecular and intramolecular interactions. A Lennard- Jones potential has been used to deal with the F-F intermolecular interactions. In the intramolecular term the flexibility of SF6 molecule has been considered by means of six harmonic stretch functions, modelling the S-F chemical bonds, and twelve harmonic bending functions, modelling the F-S-F angular deformations. The molecular dynamics simulation technique has been used to calculate the vapor-liquid coexistence curve, several thermodynamics states at the homogeneous gas and liquid region and transport coefficients of SF6 have been calculated, obtaining a good agreement with available experimental data. Diffusion coefficients and shear viscosities for SF6/N2 mixtures have also been estimated, finding good agreement with theoretical predictions for these quantities. Furthermore, the optimized SF6 force field has been used to study the process of adsorption in individual cylindrical pores by means of the Grand Canonical Monte Carlo simulations. This work demonstrates the advantages of including both types of properties in the force field development as it makes it transferable for the study of other properties.

This work is partially financed from the Spanish Government (CTQ2005-00296/PPQ) and by the Generalitat de Catalunya (2005SGR-00288). Additional support from Air Products through a MATGAS project grant is also acknowledged.

1.P. Ungerer, C. Beauvais, J. Delhommelle, A. Boutin, B. Rousseau and A. H. Fuchs. Journal of Chemical Physics 112, 5499 (2000). 2.A.Olivet and L. F. Vega. Journal of Chemical Physics. 126, 144505 (2007).