Optimized step potential models for n-alkanes and benzene

Vapor pressure, density, and internal energy data from the literature are correlated in terms of the intermolecular interactions as represented by stepwise potential energies. Discontinuous molecular dynamics simulations are performed for united atom hard chain models of ethane, n-butane, n-hexane, n-octane, and benzene. Assuming square-well attractions ending at r/σ=1.2, 1.5, 1.8, and 2.0, the depths of each well are regressed by computing the physical properties from thermodynamic perturbation theory at each set of trial depths and minimizing the deviations from experimental observation. The result is an equation-of-state for each fluid derived directly from the intermolecular potential model. Because molecular dynamics form the basis for the reference fluid simulations, transport properties may also be derived. Vapor pressure, density, and internal energy are correlated to roughly 1% average absolute deviation. The trends in the resulting potential models suggest that the change in disperse attractions with distance may vary from one functional group to another. For example, the optimized potential model for benzene diminishes to zero more slowly than the model for ethane.