Water is unique in that the experimental free energy as a function of temperature and pressure is well-known to extremely high precision. The International Association for the Properties of Water and Steam produces periodic reports on the thermophysical properties of water. Although free energies are not experimental quantities, because all standard thermodynamics properties are functions of the free energy, existing experiments for water can be used to construct a fitted model of the G(T, P ) that self-consistently agrees with the experimental properties to extremely high precision.