The CALPHAD (Calculation of Phase Diagrams) is based on mathematically formulated models, such as the quasichemical model or the sublattice model, to describe the thermodynamic properties of individual phases, such as the Gibbs free energy. The model parameters are evaluated from thermochemical data of the individual phases. The available thermodynamic data are stored in thermodynamic databases. Under typical experimental conditions of constant temperature and pressure, phase equilibrium is obtained by minimization of the Gibbs energy of a closed system. 

Reference:Spencer, P.J., (2008). A brief history of CALPHAD, Calphad 32, pp.1-8.