Molecular dynamics simulation of the free-energy expansion of the square-well fluid of short ranges

Abstract The high-temperature expansion of the free energy of a fluid of square wells, SW, is considered. The first four terms in this expansion are calculated for SW systems of variable range λσ , where σ is the diameter of the spherical hard-core. The properties were calculated via molecular dynamics, MD, for ranges 1 ≤  λ  ≤ 2.5 with special emphasis on the region of shorter ranges: λ  = 1.025, 1.050, 1.075, 1.100, 1.125 and 1.150. The principal aims are to compare these results with the previous ones obtained using the Monte Carlo, MC, method (Espindola-Heredia et al. J. Chem. Phys. 130, 024509 (2009)) that showed large statistical noise in the higher-order terms at high densities, and to provide a benchmark to check the theoretical Short-Range Expansion of the free energy of these systems. The results have been corrected to obtain the thermodynamic limit via a change of ensemble algorithm and by simulating systems with 125, 200, 500 and 1000 particles. The MD results are much smoother that the corresponding MC data and their precision allow to determine the behavior of the series for high densities. The simulation results are used to test a theory built to write the free energy for short ranges. The position of the critical point is calculated with this theory for very short ranges.