A simple approximation for fluids with narrow attractive potentials

A simple modification of the optimized random phase approximation (ORPA) has been made, aimed at improving the performance of the theory for interactions with a narrow attractive well by taking into account contributions to the direct correlation function that are nonlinear with respect to the interaction. The theory is applied to a hard core Yukawa and a square-well potential. Results for the equation of state, the correlations, and the critical point have been obtained for attractions within several ranges, and compared with Monte Carlo simulations. When the attractive interaction is narrow, the modified ORPA is a significant improvement over the plain one, especially with regard to the consistency between different routes to the thermodynamics, the two-body correlation function, and the critical temperature. However, although the spinodal curve of the modified theory is accessible, the liquid-vapour coexistence curve is not. A possible strategy to overcome this drawback is suggested.