text: The Peng–Robinson equation of state relates the three interdependent state properties pressure P, temperature T, and molar volume V.  From the state properties (P, V, T), one may compute the departure function for enthalpy per mole (denoted h) and entropy per mole (s):
  where  is defined in the Peng-Robinson equation of state, T is the reduced temperature, P is the reduced pressure, Z is the compressibility factor, and
  Typically, one knows two of the three state properties (P, V, T), and must compute the third directly from the equation of state under consideration.  To calculate the third state property, it is necessary to know three constants for the species at hand: the critical temperature T, critical pressure P, and the acentric factor ω.  But once these constants are known, it is possible to evaluate all of the above expressions and hence determine the enthalpy and entropy departures.