On the right side of Eq. 3, the first item refers to the long-distance kinetic energy of molecules, and the second item refers to the spontaneous system work that a system does due to the loss of kinetic energy. The last item is arbitrarily added to correct the discrepancy of the state equation as the temperature approaches the critical temperature, Tc. I term it the system potential. α values have been given in Ref. [29]. Inversely, the endothermal equation of a gas system is,