Fig. The defect formation energy diagram of the B vacancy V B in a hypothetical II-VI compound AB (a). The slope of the formation energy of the vacancy changes at E (+2/0) above the valence band maximum, which corresponds to a transition between the vacancy releasing 2 electrons to the electron reservoir (V +2 B ) and the vacancy retaining the 2 electrons (V 0 B ). This change in behavior can correspondingly be interpreted as populating or depopulating a molecular orbital at E (+2/0) above the valence band maximum (b), where an unfilled molecular orbital corresponds to V +2 B and a filled orbital corresponds to V 0 B .