For a vdW fluid the molar Helmholtz potential is simply f r = u r − − T r s r = C u + T r { c − − C s − − ln ⁡ ⁡ [ T r c ( 3 v r − − 1 ) ] } − − 9 / ( 8 v r ) where f r = f / ( R T c ) . A plot of this function for the suncritical isotherm T r = 7 / 8 is the one shown in Fig. 8 along with the line tangent to it at its two coexisting saturation points. The data illustrated in Fig. 8 is exactly the same as that shown in Fig.1 for this isotherm.