The large‐scale modeling of the atomic and electronic structure of KNbxTa1−xO3(KTN) perovskite solid solutions is performed using the Intermediate Neglect of the Differential Overlap (INDO) method based on the Hartree‐Fock formalism. It is found that periodic Nb impurities in KTaO3reveal coherent off‐center displacements already at the smallest calculated concentration,x=0.125. The calculated magnitude of ⟨ 111 ⟩ Nb off‐center displacement is 0.27 a.u., which is close to the XAFS observation at 70 K andx=0.09. In contrast, Ta impurities in KNbO3remain on‐center, due to higher ionicity of Ta, as compared to Nb. Using the calculated energy gain caused by the off‐center displacements of Nb atoms, the non‐empirical Ginzburg‐Landau‐Devonshire functional with concentration‐dependent coefficients is constructed. Analysis of INDO results for several Nb concentrations in KTN allows calculate the lowest critical Nb concentration,xcr=0.025, corresponding to the quantum displacive limit for the paraelectric‐ferroelectric transition at 0 K. This value is only slightly higher than the experimental one. © 2003 American Institute of Physics