A Singularity in the First-order PY Equation for a Square Well Fluid
  It is shown that a nearest nieghbor Square Well (SW) potential leads to singular behavior in first order. A solution of the first-order perturbative PY equation for an attractive nearest neighbor square well of width less than the core diameter reveals singular behavior. Neither the width of the well nor the depth of the well affects the location of the singularity in the inverse isothermal compressibility. The analysis is carried out in one dimension. 
Sheldon Shanack, Rodney Varley