SPONTANEOUS POLARIZATION AS A BERRY PHASE OF THE HARTREE-FOCK WAVE-FUNCTION - THE CASE OF KNBO3

We have investigated the ferroelectric polarization of the perovskite oxide KNbO3 within the self-consistent Hartree-Fock (HF) method, where the crystalline orbitals are expanded over a set of localized functio ns. According to the modern theory, macroscopic polarization is a geom etric quantum phase: here we show that-within the HF framework-polariz ation can be cast as a Berry phase of Slater determinants. We calculat e this observable for KNbO3 in its tetragonal phase. Besides polarizat ion, we investigate several other properties of the electronic ground state, including the broken-symmetry instability of the tetragonal str ucture. We therefore assess the reliability and the predictive power o f the HF approach when dealing with this material, which is a paradigm atic case of intermediate ionic/covalent crystal. [S0163-1829(97)03739 -9].