Liquid-gas critical point of a two-dimensional system of hard ellipses with attractive wells

In an effort to illuminate the general principles governing the critical behavior of model fluids, we investigate in this study how the shape and the (attractive) interaction range of the molecule affect the gas-liquid equilibrium and the critical behavior of the system. A combination of Monte Carlo simulations and analytical theory is employed to compute critical properties, i.e., temperature and density, of a system of hard-core ellipses with an attractive square-well potential in two-dimensional space. The critical temperature is found to decrease monotonically as the asphericity of the molecule is increased. This trend can be successfully explained in terms of the strength of the effective attraction acting between molecules measured, for instance, by the second virial coefficient. The critical density shows a complex dependence on both the range of attraction and the asphericity of the molecule. We find that the properties of particle clusters formed in near-critical states reproduce some of the most important features of the critical density, including multiple minima and maxima. It is shown that a model based on the extent of the overlap between attractive shells surrounding the ellipses captures the variation of the size of the clusters. Based on the obtained results, we discuss implications of varying the shape of the attraction potential for critical density.