The Statistical Mechanics of Condensing Systems. I

It is shown that for a system composed of N identical molecules with mutual potential energy, the assumption that the total potential energy can be expressed as the sum of that between pairs of molecules allows the derivation of simple, accurate formal equations for the thermodynamic properties of the system. Under certain conditions, generally fulfilled at low temperatures, the equations predict a region where the pressure and Gibb's free energy are independent of volume, the characteristic of condensing systems. The equations permit calculation of the Gibb's free energy of the liquid in equilibrium with the vapor and all the properties of the saturated vapor, but not the volume or volume dependence of the condensed phase.