Critical behavior of the Widom-Rowlinson mixture: coexistence diameter and order parameter

The critical behavior of the Widom-Rowlinson J. Chem. Phys. 52, 1670 (1970)] is studied in d = 3 dimensions by means of grand canonical Monte Carlo simulations. The finite-size scaling approach of Kim et al. Phys. Rev. Lett. 91, 065701 (2003)] is used to extract the order parameter and the coexistence diameter. It is demonstrated that the critical behavior of the diameter is dominated by a singular term proportional to t(1-alpha), with t the relative distance from the critical point, and alpha the critical exponent of the specific heat. No sign of a term proportional to t(2beta) could be detected, with beta the critical exponent of the order parameter, indicating that pressure mixing in this model is small. The critical density is measured to be rhosigma3 = 0.7486 +/- 0.0002, with sigma the particle diameter. The critical exponents alpha and beta, as well as the correlation length exponent nu, are also measured and shown to comply with d = 3 Ising criticality.