First-Order Perturbation Theory for Associating Fluids in the Isothermal–Isobaric Ensemble

In this study, we invert Wertheim’s first-order thermodynamic perturbation theory (TPT1) to the isothermal–isobaric ensemble (NPT) for a hard-sphere reference system. The inversion is conducted using exact relationships that connect the NVT and NPT ensembles for a hard-sphere fluid. We implement our method on spherical molecules with repulsive cores that associate through square-well attractive sites. The derived NPT expressions for the fractions of nonbonded molecules, the compressibility factor, and internal energy show excellent agreement with molecular simulation data and are also consistent with results obtained from the original TPT1. Furthermore, the proposed NPT theory is found to be superior to other associating theories.