the difference between the thermodynamic potentials F and G≡F+PV in the usual “P−V thermodynamics”. Hence, the free energy F given by Equation (???) may be understood as ...the difference between the thermodynamic potentials F and G≡F+PV in the usual “P−V thermodynamics”. Hence, the free energy F given by Equation (???) may be understood as the Gibbs energy of the Ising system in the external field, and the equilibrium value of the order parameter may be found from the last of Eqs. (1.4.16) with the replacements –P \rightarrow h, V \rightarrow N\eta :