where µ O is referenced to the total energy of an O atom in molecular oxygen, ∆µ O (T, p) = µ O (T, p) -1 2 E total O 2 , and ∆ f G(0, 0) is the Gibbs free energy of formation per formula unit of the respective bulk oxide, i.e., Cu 2 O or CuO. The lower boundary of Equation ( ) is called oxygen-poor (O-poor) limit, while the upper boundary is referred to as oxygen-rich (O-rich) limit. By assuming that the contributions of the pV term as well as the complete vibrational contribution to the Gibbs free energy can be neglected, the Gibbs free energies in Equations ( ) and ( ) can be approximated by total energies extracted from DFT calculations. This al-lows rewriting Equation ( ) and (2) as