Precise Simulation of Near-Critical Fluid Coexistence

We present a novel method to derive liquid-gas coexisting densities, ρ^±(T), from grand canonical simulations (without knowledge of T_c or criticality class). The minima of Q_L≡⟨m²⟩_L²/⟨m⁴⟩_L in an L×L×L box with m=ρ-⟨ρ⟩_L are used to generate recursively an unbiased universal finite-size scaling function. Monte Carlo data for a hard-core square-well fluid and for the restricted primitive model electrolyte yield ρ^± to ±1%–2% of ρ_c down to 1 part in 10⁴–10³ of T_c (and confirm well Ising character). Pressure mixing in the scaling fields is unequivocally revealed and indicates Yang-Yang ratios R_μ=-0.04₄ and 0.2₆ for the two models, respectively.