Derivation of a thermodynamic closure relation in the isothermal-isobaric ensemble using quasi-Gaussian entropy theory

In an analogous way as was done previously in the canonical ensemble, we derived for dilute gases an approximated thermodynamic closure relation in the isothermal-isobaric ensemble using quasi-Gaussian entropy theory. For the Gamma state, we formulated equations for the temperature dependence of various thermodynamic properties at constant pressure. The theory was applied to gaseous water and methane at 1 and 10 bar respectively, and gave for both systems an accurate prediction of the entropy and heat capacity over a large temperature interval.