  An ab initio LMTO approach and semi-empirical quantum chemical INDO method
have been used for supercell calculations of basic point defects - F-type
centers and hole polarons bound to cation vacancy - in partly covalent
perovskite KNbO3. We predict the existence of both one-site and two-site
(molecular) polarons with close absorption energies (~ 1 eV). The relevant
experimental data are discussed and interpreted.
