The new variables introduced in the previous section assume the
temperature to be defined, hence there must be thermodynamic
equilibrium in some meaningful sense. That is important for
identifying their microscopic descriptions, since the canonical
expression
Consider first the Helmholtz free energy:
For the Gibbs free energy, add
How about showing that this chemical potential is the same one as in the Maxwell-Boltzmann, Fermi-Dirac, and Bose-Einstein distribution functions for weakly interacting particles? It is surprisingly difficult to show it; in fact, it cannot be done for distinguishable particles for which the entropy does not exist. It further appears that the best way to get the result for bosons and fermions is to elaborately re-derive the two distributions from scratch, each separately, using a new approach. Note that they were already derived twice earlier, once for given system energy, and once for the canonical probability distribution. So the dual derivations in {D.61} make three. Please note that whatever this book tells you thrice is absolutely true.