Accurate first-order perturbation theory for fluids:<i>uf</i>-theory

We propose a new first-order perturbation theory that provides near-quantitative description of the thermodynamics simple fluids. The is based on ansatz Helmholtz free energy bounded below by Mayer-f expansion. Together with rigorous upper bound provided u-expansion, this brackets actual between an and (effective) lower can both be calculated theory. This great practical use. Here, two bounds are combined into interpolation scheme for energy. exploits fact exact in low-density limit, whereas accuracy u-expansion grows when density increases. allows “f”-bound at low densities “u” higher liquid-like densities. resulting particularly well behaved. Using density-dependent interpolating function only adjustable parameters, we obtain very accurate representation full fluid-phase behavior Lennard-Jones fluid. transferable to other intermolecular potential types, which here shown Mie m-6 family extension mixtures without requiring any dependence composition mixture.