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). formula_1 is the number of particles of type "i" in the system. For the sake of completeness, the set of all formula_1 are also included as natural variables, although they are sometimes ignored.