The Fisher-Widom line for a hard core attractive Yukawa fluid

The Fisher-Widom (FW) line is the locus of points in the density-temperature (rho, T) plane where the longest range decay of the pairwise correlation function of a fluid crosses over from monotonic to damped oscillatory decay. The FW line for a hard-core potential with an attractive Yukawa tail is calculated using two integral equation theories, the random phase approximation and the mean spherical approximation. Although the pole structure of the structure factor in this fluid is similar to that determined for model fluids with finite ranged potentials, the FW line is shifted far from the critical point and intersects the liquid branch of the coexistence curve at a much lower reduced temperature T/T-c than is found for the square-well or truncated Lennard-Jones models. The dependence of the FW line on the choice of model and approximate theory is discussed.