In this work, the analytical SAFT-γ Mie equation of state (EoS) is used in the efficient development of intermolecular force field parameters which can be used in direct molecular simulation for a range of fused dimer molecules.
In its original formulation, the SAFT-γ Mie EoS only provides a direct link to the force field parameters for tangent models. A novel development here is to extend the application of the equation of state to obtain intermolecular potentials for fused models which more faithfully mimic realistic molecules. The proposed methodology consists of a two stage process:
- An empirical mapping between the parameters of the theory and parameters used in simulation is first obtained by performing molecular dynamics simulations of the vapour-liquid equilibria for a wide range of homonuclear model dimer molecules with different bond lengths and Mie repulsive exponents. The simulation results are matched with those obtained using the theory for corresponding systems in order to determine the relationship between the parameters of the theory and the potential model. A methodology for obtaining unlike interaction parameters in heteronuclear dimer molecules is also developed and validated.
- The mapping obtained is then used to translate the equation of state parameters of real molecules into a simulation force field with an explicit bond length.
Force fields are obtained for a range of molecules with different levels of molecular resolution including atomistic models for molecular oxygen and nitrogen, united-atom models for ethane and perfluoroethane and more coarse-grained dimer models of propylbenzene and carbon dioxide. The properties obtained by simulation include vapour pressures, saturated liquid densities and interfacial tensions. This top-down methodology of obtaining force field parameters for computer simulation of fluids is
much less computer and time intensive than the traditional methods. In the concluding chapter, a preliminary extension of the approach to trimers molecules is presented.