Ab initio electronic structure calculations using the density-functional theory (DFT) are performed for KNbO_3 with and without defects. Ferroelectric distortive transitions involve very small changes in energies and are therefore sensitive to DFT-approximations. This is discussed by comparing results obtained with the local density approximation (LDA) to those where generalized gradient approximations (GGA) are used. The results of ab initio calculations for F-type centers and bound hole polarons are compared to those obtained by a semiempirical method of the Intermediate Neglect of the Differential Overlap (INDO), based on the Hartree-Fock formalism. Supercells with 40 and 320 atoms were used in these two approaches, respectively. The relevant experimental data are discussed.
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