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We develop Wertheim's Thermodynamic Perturbation Theory, TPT, to treat fluid dimers made of square-well, SW, atoms [M.S. Wertheim, J. Chem. Phys.  85 , 2929 (1986)]. In this, we follow the general scheme provided by the Statistical Associating Fluid Theory, SAFT [W. G. Chapman  et al. , Ind. Eng. Chem. Res.  29 , 2709 (1990)]. The theoretical results are calibrated by dealing with the dimer with dimensionless SW range, and then considering. The TPT-SAFT free energy contains two essential contributions: a SW atomic term and a dimerisation term. The SW atomic term is taken to fourth-order in a high-temperature expansion, HTE. The dimerisation contribution is analysed to determine the extent to which Wertheim's approach accounts correctly for the change in SW free energy. This contribution relies on the cavity function of two particles at contact,, that is expressed in terms of density, temperature and  λ  by correlations of published Monte Carlo data, implying a fourth-order semi-empirical HTE. The SW dimer fluid-phase behaviour is confronted with published Monte Carlo data. The TPT-SAFT here presented accounts very well for the one-phase and liquid-vapour coexistence properties, except for a small region around the critical point .

Thermodynamic perturbation theory of square-well dimers of variable width