Thermodynamic potential
where T = temperature, S = entropy, p = pressure, V = volume. The Helmholtz free energy is often denoted by the symbol F, but the use of A is preferred by IUPAC (See Alberty, 2001).  is the number of particles of type i in the system. For the sake of completeness, the set of all  are also included as natural variables, although they are sometimes ignored.