Global square-well free-energy model via singular value decomposition

ABSTRACT

The free energy of the square-well (SW) fluid is a quantity of great interest in thermodynamics and statistical mechanics. Several authors have calculated by simulations the first four terms in the high-temperature perturbation expansion for SW ranges from λ = 1 up to λ = 2.5 or 3. Besides, the asymptotic form of the first two terms in the expansion, for λ large, is known analytically. The information gathered so far seems to indicate that a range of λ = 3 is not far from this asymptotic van der Waals regime. In this work, we use the technique of singular value decomposition (SVD) to provide us with expressions of the SW free energy valid for all ranges of 1 ≤ λ and, for the higher-order terms, covering the density range upto the random close-packed state. Besides rendering unified expressions of the free energy for all ranges, the SVD allows us to separate the perturbation terms into a sum of products of functions of density and range, so that one can discern the most important contributions and extract the underlying density and range profiles.