The results of supercell calculations of electronic structure and related properties of substitutional impurities in perovskite oxides KNbO_3 and KTaO_3 are discussed. For Fe impurities in KNbO_3, the results obtained in the local density approxiamtion (LDA) and in the LDA + U approach (that allows an ad hoc treatment of nonlocality in exchange-correlation) are compared, and different impurity charge configurations are discussed. The study of off-centre Li defects in incipient ferroelectric KTaO_3 have been done by the appropriately parametrized Intermediate Neglect of Differential Overlap (INDO) method. The interaction energies of two off-centre impurities in different relative configurations are discussed.
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