A simple approximation for fluids with narrow attractive potentials

We study a simple modification of the optimized random phase approximation (ORPA) 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 non-linear in 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 of several ranges, and compared with Monte Carlo simulations. When the attractive interaction is narrow, the modified ORPA significantly improves 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, while the spinodal curve of the modified theory is accessible, the liquid-vapor coexistence curve is not. A possible strategy to overcome this drawback is suggested.