Precise Simulation of Near-critical Fluid Coexistence

We present a novel method to derive liquid-gas coexisting densities, ${\rho }^{\pm }(T)$, from grand canonical simulations (without knowledge of $\text{Tc}$ or criticality class). The minima of $ Q_{L}\equiv< m^{2} >{L}^{2}/< m^{4}>{L}$inan$L$$\times$$L \times$$L$ box with $m=\rho -<\rho {>}_L$ 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 ${\rho }^{\pm }$ to $\pm 1$-2% of $\text{rhoc}$ down to 1 part in ${10}^4$-${10}^3$ of $\text{Tc}$ (and confirm well Ising character). Pressure mixing in the scaling fields is unequivocally revealed and indicates Yang-Yang ratios $R_{\mu }=-{0.04}_4$ and ${0.2}_6$ for the two models, respectively.