The set of natural variables for each of the above four thermodynamic potentials is formed from a combination of the T, S, p, V variables, excluding any pairs of conjugate variables ; there is no natural variable set for a potential including the T - S or p - V variables together as conjugate variables for energy. An exception for this rule is the N i − μ i conjugate pairs as there is no reason to ignore these in the thermodynamic potentials, and in fact we may additionally define the four potentials for each species. Using IUPAC notation in which the brackets contain the natural variables (other than the main four), we have: