Perturbation theory theoretical background

High-pressure phase equilibria in systems of polymers, solvents, and supercritical gases are in almost all cases modeled using equations of state. A review of equations of state for polymer systems, including a discussion of their theoretical background, has been given by Lambert et al. [6]. One of the first equations of state that was used to model the high-pressure phase behavior of polymer-solvent systems was the Flory equation of state [11, 12]. Patterson and Delmas [10] showed that this equation of state can be used to describe both LCST and UCST phase behavior. The perturbed hard-chain theory (PCHT) was developed by Prausnitz and coworkers [57-59]. It can be considered as an improvement of the approach of Flory... 

In this section, we briefly survey the basic theoretical apparatus of multi-reference second-order Brillouin-Wigner perturbation theory. This will serve both to provide the necessary background and to introduce our notation. 

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