Spontaneous polarization as a Berry phase of the Hartree-Fock wave function: The case of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">KNbO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

We have investigated the ferroelectric polarization of the perovskite oxide ${\mathrm{KNbO}}_{3}$ within the self-consistent Hartree-Fock (HF) method, where the crystalline orbitals are expanded over a set of localized functions. According to the modern theory, macroscopic polarization is a geometric quantum phase: here we show that---within the HF framework---polarization can be cast as a Berry phase of Slater determinants. We calculate this observable for ${\mathrm{KNbO}}_{3}$ in its tetragonal phase. Besides polarization, we investigate several other properties of the electronic ground state, including the broken-symmetry instability of the tetragonal structure. We therefore assess the reliability and the predictive power of the HF approach when dealing with this material, which is a paradigmatic case of intermediate ionic/covalent crystal.

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