Si . Although the charge state of P Si could be understood simply from valence electron counting, this is not true in general since the placement of the excess charge (either near the defect site or in the electron reservoir) depends on the energy of the electron reservoir (i.e., the Fermi level E F ). In fact, a point defect may possess multiple charge states depending on the position of E F . A famous example of this is the oxygen vacancy (V O ) in ZnO, where computational studies and EPR experiments have asserted that the charge state is neutral (i.e., V 0 O ) when E F is near the conduction band minimum, and the charge state is +2 (i.e. V +2 O ) when E F is near the valence band maximum.