Screening in ionic systems: Simulations for the Lebowitz length

Simulations of the Lebowitz length, ξL(T,ρ), are reported for the restricted primitive model hard-core (diameter a) 1 1 electrolyte for densities ρ 4ρc and TcT 40Tc. Finite-size effects are elucidated for the charge fluctuations in various subdomains that serve to evaluate ξL. On extrapolation to the bulk limit for T 10Tc the exact low-density expansions are seen to fail badly when ρ>110ρc (with ρca3 0.08). At higher densities ξL rises above the Debye length, ξD T/ρ, by 10%-30% (up to ρ 1.3ρc); the variation is portrayed fairly well by the generalized Debye-Hückel theory. On approaching criticality at fixed ρ or fixed T, ξL(T,ρ) remains finite with ξLc 0.30a 1.3ξDc but displays a weak entropylike singularity.