  First-principles modeling of ferroelectric capacitors presents several
technical challenges, due to the coexistence of metallic electrodes, long-range
electrostatic forces and short-range interface chemistry. Here we show how
these aspects can be efficiently and accurately rationalized by using a
finite-field density-functional theory formalism in which the fundamental
electrical variable is the displacement field D. By performing calculations on
model Pt/BaTiO3/Pt and Au/BaZrO3/Au capacitors we demonstrate how the
interface-specific and bulk-specific properties can be identified and
rigorously separated. Then, we show how the electrical properties of capacitors
of arbitrary thickness and geometry (symmetric or asymmetric) can be readily
reconstructed by using such information. Finally, we show how useful
observables such as polarization and dielectric, piezoelectric and
electrostrictive coefficients are easily evaluated as a byproduct of the above
procedure. We apply this methodology to elucidate the relationship between
chemical bonding, Schottky barriers and ferroelectric polarization at
simple-metal/oxide interfaces. We find that BO2-electrode interfaces behave
analogously to a layer of linear dielectric put in series with a bulk-like
perovskite film, while a significant non-linear effect occurs at AO-electrode
interfaces.
