Thermodynamic potentials are functions that characterize the equilibrium state of a substance. An example is the Gibbs free energy , which is a function of pressure and temperature. Knowing any one thermodynamic potential  is sufficient to compute all equilibrium properties of a substance, often simply by taking derivatives of . Thus, a single correlation for  can replace separate correlations for individual properties. Conversely, a variety of experimental measurements (e.g., density, heat capacity, vapor pressure) can be incorporated into the same fit; in principle, this would allow one to predict hard-to-measure properties like heat capacity in terms of other, more readily available measurements (e.g., vapor pressure).