Hartree-Fock studies of surface properties of BaTiO₃

We study several surface properties of BaTiO₃: namely, the surface charge, surface energy, and surface dynamical charges, within an ab initio Hartree-Fock scheme. We use both genuinely isolated slabs and periodically repeated slabs, with different terminations. We report these properties as functions of the thickness and termination of the slabs. The ability to deal with genuinely isolated slabs is a virtue of the localized-basis implementation that we adopt. In particular, the slab surface charge provides a value for the spontaneous polarization in excellent agreement with the calculation of the latter as a Berry phase, and with experiment as well. On the other hand, we find that when periodically repeated slabs are used, the interactions among slabs and the fictitious field imposed by the periodic boundary conditions can significantly affect the accuracy of the calculations, even at quite large vacuum separations.