Ising criticality in fluids: A counterexample

It is commonly accepted that fluids undergoing a liquid-vapor type transition belong to the universality class of the Ising model. In this paper, for a two-dimensional system of point particles, interacting via short-ranged pair potentials, and featuring two fluid phases (i.e. distinghuished by a density gap), we show this to be not the case. Instead, at the critical point, $\beta/\nu \sim 0.5$ and $\gamma/\nu=1.0$ are observed, which are distinctly non-Ising ($\beta,\gamma,\nu$ being the critical exponents of the order parameter, susceptibility, and correlation length, respectively). Paramount in our analysis is a scheme to facilitate the extrapolation of simulation data in parameters that are not necessarily field variables, the details of which are also provided.