The Relationship Between the Hard Sphere Fluid and Fluids with Realistic Repulsive Forces
CALIFORNIA UNIV SAN DIEGO LA JOLLA DEPT OF CHEMISTRY
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The study considers the equilibrium statistical mechanics of classical fluids in which the potential energy is resolvable into repulsive pair interactions. A generalized cluster expansion is derived relating the thermodynamic and structural properties of such systems to those of the hard sphere fluid. The expansion is ordered by a softness parameter which is essentially the range of intermolecular distances in which the difference between the Mayer functions for the repulsive potential and an appropriate reference hard sphere potential is non-zero. The first lowest order approximation generated by the expansion equates the free energy and distribution function for the fluid to the respective functions appropriate to a system of hard spheres with diameter d. A prescription is given for choosing a temperature and density dependent diameter d of the reference hard sphere fluid so that the first approximation for the free energy contains errors of one order only. With this approximation, the results obtained for both model systems agree closely with those obtained by Monte Carlo calculations.
- Atomic and Molecular Physics and Spectroscopy