Based on the modified local composition, a new coordination number model is developed. The main features of the model are its simplicity in form, good accuracy in representation of the simulation data of square-well fluids and consistency with both low and high density boundary conditions. Using the new coordination number, a new expression for the attractive part of square-well equation of state (SWEOS) is derived. The two types of SWEOS are introduced by sum of the new attractive term and the previous repulsive expressions of Carnahan–Starling and van der Waals. The adjustable parameters of SWEOS are the square-well potential parameters: σ, R and ɛ/k. The new SWEOSs are applied for the correlation of the vapor pressure, liquid density and prediction of the vapor molar volume of 38 pure compounds. The results demonstrate that the new SWEOSs are successful to describe the thermophysical properties of a wide variety of fluids with very good accuracy.

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