We consider a simple pure substance under hydrostatic conditions
described by the following fundamental equation: where the
extensive variables U, V and N are the internal energy, the
volume, and the number of particles respectively, and the
intensive variables T, p and are the temperature, the pressure
and the chemical potential respectively. Equation () corresponds
to the choice of the variables U, V and N as independent
variables of the entropy S(U,V,N). These variables are precisely
those which are fixed and determine the macrostate of the
members of the Microcanonical Ensemble and consequently S is the
relevant potential in this statistical ensemble. It is useful to
define the following quantities: , and so that Eq. () can then
be written in the dimensionless form: In general, for other
thermodynamic systems with degrees of freedom, one will have:
where are extensive variables, and the corresponding entropic
conjugate variables. Massieu-Planck functions  are entropic
thermodynamic potentials defined as L