Journal
JOURNAL OF CHEMICAL PHYSICS
Volume 154, Issue 4, Pages -Publisher
AMER INST PHYSICS
DOI: 10.1063/5.0031545
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Funding
- German Research Foundation (DFG) within the Cluster of Excellence in Simulation Technology at the University of Stuttgart [EXC 2075]
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The new first-order perturbation theory provides a near-quantitative description of thermodynamics for simple fluids, with constraints on the actual free energy between lower and upper bounds calculated based on first-order perturbation theory. The theory allows accurate interpolation between low and high densities, with a transferable interpolating function for different intermolecular potential types.
We propose a new first-order perturbation theory that provides a near-quantitative description of the thermodynamics of simple fluids. The theory is based on the ansatz that the Helmholtz free energy is bounded below by a first-order Mayer-f expansion. Together with the rigorous upper bound provided by a first-order u-expansion, this brackets the actual free energy between an upper and (effective) lower bound that can both be calculated based on first-order perturbation theory. This is of great practical use. Here, the two bounds are combined into an interpolation scheme for the free energy. The scheme exploits the fact that a first-order Mayer-f perturbation theory is exact in the low-density limit, whereas the accuracy of a first-order u-expansion grows when density increases. This allows an interpolation between the lower f-bound at low densities and the upper u bound at higher liquid-like densities. The resulting theory is particularly well behaved. Using a density-dependent interpolating function of only two adjustable parameters, we obtain a very accurate representation of the full fluid-phase behavior of a Lennard-Jones fluid. The interpolating function is transferable to other intermolecular potential types, which is here shown for the Mie m-6 family of fluids. The extension to mixtures is simple and accurate without requiring any dependence of the interpolating function on the composition of the mixture.
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