Ultimately, the stability of a given phase is determined from its free energy, G, in relation to its competing phases . Stability in the solid state is often determined from the internal energy or enthalpy based on density functional theory (DFT) calculations, but when comparing phase stability at di↵erent temperatures, en-⇤ k.tolborg@imperial.ac.uk † a.walsh@imperial.ac.uk tropic contributions must be taken into account.