An optimized molecular force field for sulfur hexafluoride (SF6) simulations is presented in this work. The new force field for SF6 contains two parts: a Lennard-Jones potential that deals with F-F intermolecular interactions and the second term dealing with the intramolecular forces. In this second part the flexibility of the molecule is explicitly considered by 6 harmonic stretch terms, modeling the S-F chemical bonds, and 12 harmonic bending terms, modeling the F-S-F angular deformations. The parameters of the new force field have been obtained by a multivariable optimization procedure, whose main feature is the simultaneous fitting of all force field parameters, using as reference data several equilibrium properties (vapor pressure, saturated liquid density, and surface tension) and shear viscosity. The new force field clearly improves the description of the phase envelope and the rest of the properties as compared to previous simulations for a rigid model for the same molecule [A. Olivet et al., J. Chem. Phys. 123, 194508 (2005)]. Results for the optimized force field concerning the vapor-liquid coexistence curve, several thermodynamics states at the homogeneous gas and liquid region, and transport coefficients of SF6 are in good agreement with available experimental data.

We present and discuss results concerning electron interactions processes of dielectric gases and their relationship with the macroscopic behavior of these gases, in particular, with their dielectric strength. Such analysis is based on calculating energies of reactions for molecular ionization, dissociative ionization, parent negative ion formation, and dissociative electron attachment processes. We hypothesize that the estimation of the required energy for a reduced number of processes that take place in electrically stressed gases could be related to the gas' capability to manage the electron flow during an electrical discharge. All calculations were done with semiempirical quantum chemistry methods, including an initial optimization of molecular geometry and heat of formation of the dielectric gases and all of species that appear during electron interaction reactions. The performance of semiempirical methods Austin model 1 and Parametric model 3 ͑PM3͒ was compared for several compounds, PM3 being superior in most cases. Calculations performed for a sample of nine dielectric gases show that electron attachment and detachment processes occur in different energy bands that do not overlap for any value of the dielectric strength. We have also analyzed the relationship between dielectric strength and two physical properties: electron affinity and ionization energy. Calculations performed for 43 dielectric gases show no clear correlation between them, although certain guidelines for the qualitative estimation of dielectric strength can still be assessed.

We present and discuss here results concerning the implementation of molecular dynamics simulations with the Green−Kubo formalism to predict transport coefficients in SF6/N2 mixtures under conditions of practical interest. SF6 was modeled using a flexible molecular force field recently proposed [Olivet, A.; Vega, L. F. J. Chem. Phys. 2007, 126, 144502], while the force field of Galassi and Tildesley was used for N2 modeling [Galassi, G.; Tildesley, D. J. Mol. Simul. 1994, 13, 11]. The influence of mixture composition on the mutual diffusion coefficients and shear viscosities was investigated by a series of molecular dynamic simulations of SF6/N2 mixtures with different compositions at 300 K and 1 MPa. Temperature and pressure dependencies of the mutual diffusion coefficient were also investigated for a fixed composition of SF6/N2, covering the temperature range from 260 to 340 K and two pressure values (1 and 2 MPa). Simulations with these anisotropic force fields led to mutual diffusion coefficients that diminish when the SF6 concentration in the mixture is increased. This behavior differs from the hypothesis of no composition dependence assumed for several estimation methods available in the literature for gaseous binary mixtures at low pressures. Such a composition dependence is explained on the basis of the deviations from ideality of SF6/N2 mixtures. The effect of the molecular flexibility considered in the SF6 molecular model is analyzed by comparing the transport coefficients estimated with the aforementioned force fields to those estimated with simple Lennard-Jones potentials. Differences between predictions with the anisotropic force fields and the simpler Lennard-Jones potentials were more significant in the case of the shear viscosity; while viscosities estimated with the Lennard-Jones potentials showed no dependence on composition, the estimations based on the anisotropic force fields exhibited a maximum value for the viscosity, which is the same behavior described by an empirical method. Results presented here are pure predictions, thus covering part of the lack of information existing about the transport coefficients of these mixtures, which is of relevance for several industrial processes.