Universality Class of Criticality in the Restricted Primitive Model Electrolyte

The 1:1 equisized hard-sphere electrolyte or restricted primitive model has been simulated via grand-canonical fine-discretization Monte Carlo. Newly devised unbiased finite-size extrapolation methods using loci in the temperature-density or $(T,\rho )$ plane of isothermal ${\rho }^{2-k}$ vs pressure inflections, of $Q\equiv 〈m^2{〉}^2/〈m^4〉$ maxima, and of canonical and $C_V$ criticality, yield estimates of $(T_c,{\rho }_c)$ to $\pm (0.04,3)\%$. Extrapolated exponents and $Q$ ratio are $(\gamma ,\nu {,Q}_c)=\left[1.24\right(3),0.63(3);0.624(2\left)\right]$, which support Ising $(n=1\mathrm{})$ behavior with $({1.23}_9,{0.630}_3;{0.623}_6)$, but exclude classical, $\mathrm{XY}$ $(n=2\mathrm{})$, self-avoiding walk $(n=0\mathrm{})$, and $n=1\mathrm{}$ criticality with potentials $\varphi \left(r\right)>\Phi {/r}^{4.9}$ when $r\to \infty$.