  We calculate, through Monte Carlo numerical simulations, the partial total
and direct correlation functions of the three dimensional symmetric
Widom-Rowlinson mixture. We find that the differences between the partial
direct correlation functions from simulation and from the Percus-Yevick
approximation (calculated analytically by Ahn and Lebowitz) are well fitted by
Gaussians. We provide an analytical expression for the fit parameters as
function of the density. We also present Monte Carlo simulation data for the
direct correlation functions of a couple of non additive hard sphere systems to
discuss the modification induced by finite like diameters.
