Hartree–Fock Studies of the Ferroelectric Perovskites

Within an ab–initio HF scheme, we use both Berry–phase calculations and supercell calculations in order to compute the dynamical charges for lattice dynamics and the electronic dielectric constant for KNbO3 and BaTiO3. Comparison with experimental data indicates that HF provides a description of the electronic properties of this material whose accuracy is of the same order as the LDA one. There are however significant differences between the two sets of results, whose origin is scrutinized. Motivated by the study of surface and domain–boundary properties, we also present some results for BaTiO3 slabs, including both genuinely isolated and periodically repeated slabs with different terminations. The capability of dealing with a genuinely isolated slab is a virtue of the localized–basis implementation adopted here. We demonstrate, amongst other things, the nontrivial dynamical–charge neutrality of BaTiO3 [001] surfaces.