THERMODYNAMIC, MECHANICAL AND TRANSPORT-PROPERTIES OF FLUIDS WITH STEEPLY REPULSIVE POTENTIALS

Hard sphere perturbation theory expressions for the thermodynamic prop erties and the infinite frequency elastic moduli of fluids interacting with steeply repulsive pair potentials with the analytical form phi(r ) = epsilon(sigma/r)(n) where epsilon and sigma set the energy and dis tance scales, respectively, are tested against extensive molecular dyn amics (MD) simulation data. The convergence of these expressions as a function of the softness parameter n(-1) is examined by comparing with virtually exact values obtained from MD simulations of fluids interac ting with these potentials. The value of the parameter n in the simula tions ranged from 18 to the unusually high value of 288. Perturbation theory reproduces the thermodynamic properties and the infinite freque ncy elastic moduli from simulation, within the MD statistical uncertai nty for n greater than 36. The self-diffusion coefficient D and shear viscosity eta(s) were determined also and are found to be quite sensit ive to the value of n in the range studied. The convergence towards th e hard sphere value is nonlinear in n(-1) for D at high fluid densitie s. At high densities the shear stress autocorrelation function decays increasingly rapidly with time, and the associated shear stress relaxa tion time diminishes according to n(-1) in the hard sphere limit, as p redicted by perturbation theory using the Barker-Henderson equivalent hard sphere diameter.