One of the most important features of quantitative phase field models is that the equilibrium order parameter profiles decouple from concentration, or any other diffusing field to which the order parameters are coupled. This makes it possible to specify interface energy independently of the solute profiles through the interface, a practical feature for incorporating the results of microscopic studies of surface energy. Two-phase equilibrium between liquid and/or solid phases are defined when the chemical potentials reach a constant. The chapter describes two approaches for computing the solid-solid interface energy: basic model, and modification of model. The choice of the two model grand potential energy functionals offers different methods for simulating solid-solid interface energies post-solidification.