Monte Carlo simulations and perturbation theory for highly correlated fluids: The Lennard-Jones core softened potential case

Abstract

The growing interest in anomalous thermodynamic properties of isotropic fluids has been followed by several attempts to implement an accurate and simple theoretical description of these phenomena. Here, we have performed an extensive Monte Carlo exploration of the thermodynamic properties and vapor-liquid equilibrium of the Lennard-Jones core softened potential with variable Gaussian tail strengths λ spanning fluids with short range attraction and long range repulsion to those with short range repulsion and long range attraction. The results have been compared with the theoretical framework provided by the Barker and Henderson perturbation theory evaluated at longer ranges than those usually employed in the literature. The correlation in close shells particles is also incorporated in an empirical way. The agreement between the theoretical approach and the simulation results are in quantitative agreement in most cases, but worsens when the Gaussian tail is deep enough to provoke substantial modifications in the fluid structure. These structured radial distribution functions differ from those of typical atomic fluids in the liquid state and are mainly responsible of the inaccuracy of the perturbative approach for quantitative purposes. Despite this drawback, we have successfully applied the theoretical treatment for predicting anomalies in fluid models that are in agreement with those reported in the literature. These results open the door for a wide use of perturbative treatments for predicting fluid anomalies if care is taken of the range of the perturbative approach and of the particle correlation within neighboring shells.