We present first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of an oxygen vacancy in KH 2 PO 4 ͑KDP͒. Even though the overall density of states profiles for the defective KDP are quite similar to those of the perfect KDP, the oxygen vacancy in the neutral and +1 charge states induces defect states in the band gap. For the neutral oxygen vacancy, one electron is localized in the O vacancy maintaining the charge that was at

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... that site in the pure crystal, while the neighboring P atom to the vacancy gains about one electron relative to its charge state in pure KDP. For the +1 charge oxygen vacancy, the removal of an electron reduces the occupation of the filled gap states in the neutral case from two to one electron and produces a new empty state in the gap. The new empty gap state is very close to the highest occupied states, leading to a dramatic decrease of the band gap. In sharp contrast, no defect states appear in the energy gap for the +2 charge O vacancy, where the neighboring P atom to the O vacancy almost returns to its perfect-crystal charge state. For the −1 charge oxygen vacancy, the added electron does not localize on any of the atoms near the vacancy; rather it goes into the vacancy to spin-pair with the electron that was there in the neutral vacancy.