B and E Perfect are the energies of AB with and without V +2 B , respectively, µ B is the atomic chemical potential of atom B, and E F is the electron chemical potential (i.e., the Fermi level). Notice that the formation energy of V +2 B depends on E F , since 2 electrons leave the vacancy site as "reaction products". This is not true for the neutral charge V 0 B defect, where no relative change in the number of electrons occurs. Consequently, the formation energy of V 0 B is independent of E F :