\begin{thebibliography}{10}
\expandafter\ifx\csname url\endcsname\relax
  \def\url#1{\texttt{#1}}\fi
\expandafter\ifx\csname urlprefix\endcsname\relax\def\urlprefix{URL }\fi
\providecommand{\bibinfo}[2]{#2}
\providecommand{\eprint}[2][]{\url{#2}}

\bibitem{Dawber:2005}
\bibinfo{author}{Dawber, M.}, \bibinfo{author}{Rabe, K.~M.} \&
  \bibinfo{author}{Scott, J.~F.}
\newblock \bibinfo{title}{Physics of thin-film ferroelectric oxides}.
\newblock \emph{\bibinfo{journal}{Rev. Mod. Phys.}}
  \textbf{\bibinfo{volume}{77}}, \bibinfo{pages}{1083} (\bibinfo{year}{2005}).

\bibitem{Hwang:2002}
\bibinfo{author}{Hwang, C.~S.}
\newblock \bibinfo{title}{Thickness-dependent dielectric constant of
  {(Ba,Sr)TiO$_3$} thin films with {Pt} or conducting oxide electrodes.}
\newblock \emph{\bibinfo{journal}{J.~Appl. Phys.}}
  \textbf{\bibinfo{volume}{92}}, \bibinfo{pages}{432} (\bibinfo{year}{2002}).

\bibitem{Plonka:2005}
\bibinfo{author}{Plonka, R.}, \bibinfo{author}{Dittmann, R.},
  \bibinfo{author}{Pertsev, N.~A.}, \bibinfo{author}{Vasco, E.} \&
  \bibinfo{author}{Waser, R.}
\newblock \bibinfo{title}{Impact of the top-electrode material on the
  permittivity of single-crystalline {Ba$_{0.7}$Sr$_{0.3}$TiO$_3$} thin films.}
\newblock \emph{\bibinfo{journal}{Appl. Phys. Lett.}}
  \textbf{\bibinfo{volume}{86}}, \bibinfo{pages}{202908}
  (\bibinfo{year}{2005}).

\bibitem{Kim_et_al:2005}
\bibinfo{author}{Kim, D.~J.} \emph{et~al.}
\newblock \bibinfo{title}{Polarization relaxation induced by a depolarization
  field in ultrathin ferroelectric {BaTiO$_3$} capacitors}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. Lett.}}
  \textbf{\bibinfo{volume}{95}}, \bibinfo{pages}{237602}
  (\bibinfo{year}{2005}).

\bibitem{Junquera_review:2008}
\bibinfo{author}{Junquera, J.} \& \bibinfo{author}{Ghosez, P.}
\newblock \bibinfo{title}{First-principles study of ferroelectric oxide
  epitaxial thin films and superlattices: role of the mechanical and electrical
  boundary conditions}.
\newblock \emph{\bibinfo{journal}{arXiv:0711.4201}}  (\bibinfo{year}{2007}).

\bibitem{Junquera/Ghosez:2003}
\bibinfo{author}{Junquera, J.} \& \bibinfo{author}{Ghosez, P.}
\newblock \bibinfo{title}{Critical thickness for ferroelectricity in perovskite
  ultrathin films}.
\newblock \emph{\bibinfo{journal}{Nature}} \textbf{\bibinfo{volume}{422}},
  \bibinfo{pages}{506--509} (\bibinfo{year}{2003}).

\bibitem{Pertsev:2007}
\bibinfo{author}{Pertsev, N.~A.}, \bibinfo{author}{Dittmann, R.},
  \bibinfo{author}{Plonka, R.} \& \bibinfo{author}{Waser, R.}
\newblock \bibinfo{title}{Thickness dependence of the intrinsic dielectric
  response and apparent interfacial capacitance in ferroelectric thin films.}
\newblock \emph{\bibinfo{journal}{J.~Appl. Phys.}}
  \textbf{\bibinfo{volume}{86}}, \bibinfo{pages}{202908}
  (\bibinfo{year}{2005}).

\bibitem{Black/Welser:1999}
\bibinfo{author}{Black, C.~T.} \& \bibinfo{author}{Welser, J.~J.}
\newblock \bibinfo{title}{Electric-field penetration into metals: Consequences
  for high-dielectric constant capacitors}.
\newblock \emph{\bibinfo{journal}{IEEE Trans. Electron Devices}}
  \textbf{\bibinfo{volume}{46}}, \bibinfo{pages}{776} (\bibinfo{year}{1999}).

\bibitem{Tagantsev:2006}
\bibinfo{author}{Tagantsev, A.~K.} \& \bibinfo{author}{Gerra, G.}
\newblock \bibinfo{title}{Interface-induced phenomena in polarization response
  of ferroelectric thin films}.
\newblock \emph{\bibinfo{journal}{J.~Appl. Phys.}}
  \textbf{\bibinfo{volume}{100}}, \bibinfo{pages}{051607}
  (\bibinfo{year}{2006}).

\bibitem{Rabe_strain}
\bibinfo{author}{Rabe, K.~M.}
\newblock \bibinfo{title}{Theoretical investigations of epitaxial strain
  effects in ferroelectric oxide thin films and superlattices}.
\newblock \emph{\bibinfo{journal}{Curr. Opin. Solid State Mat. Sci.}}
  \textbf{\bibinfo{volume}{9}}, \bibinfo{pages}{122} (\bibinfo{year}{2005}).

\bibitem{Ederer/Spaldin_3:2005}
\bibinfo{author}{Ederer, C.} \& \bibinfo{author}{Spaldin, N.~A.}
\newblock \bibinfo{title}{Effect of epitaxial strain on the spontaneous
  polarization of thin film ferroelectrics}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. Lett.}}
  \textbf{\bibinfo{volume}{95}}, \bibinfo{pages}{257601}
  (\bibinfo{year}{2005}).

\bibitem{Sai/Rappe:2005}
\bibinfo{author}{Sai, N.}, \bibinfo{author}{Kolpak, A.~M.} \&
  \bibinfo{author}{Rappe, A.~M.}
\newblock \bibinfo{title}{Ferroelectricity in ultrathin perovskite films.}
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{72}},
  \bibinfo{pages}{020101(R)} (\bibinfo{year}{2005}).

\bibitem{Elsasser:2006}
\bibinfo{author}{Umeno, Y.}, \bibinfo{author}{Meyer, B.},
  \bibinfo{author}{{Els\"asser}, C.} \& \bibinfo{author}{Gumbsch, P.}
\newblock \bibinfo{title}{Ab initio study of the critical thickness for
  ferroelectricity in ultrathin {Pt/PbTiO3/Pt} films}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{74}},
  \bibinfo{pages}{060101(R)} (\bibinfo{year}{2006}).

\bibitem{Gerra:2006}
\bibinfo{author}{Gerra, G.}, \bibinfo{author}{Tagantsev, A.~K.},
  \bibinfo{author}{Setter, N.} \& \bibinfo{author}{Parlinski, K.}
\newblock \bibinfo{title}{Ionic polarizability of conductive metal oxides and
  critical thickness for ferroelectricity in {BaTiO3}}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. Lett.}}
  \textbf{\bibinfo{volume}{96}}, \bibinfo{pages}{107603}
  (\bibinfo{year}{2006}).

\bibitem{Gerra:2007}
\bibinfo{author}{Gerra, G.}, \bibinfo{author}{Tagantsev, A.~K.} \&
  \bibinfo{author}{Setter, N.}
\newblock \bibinfo{title}{Ferroelectricity in asymmetric
  metal-ferroelectric-metal heterostructures: A combined
  first-principles--phenomenological approach}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. Lett.}}
  \textbf{\bibinfo{volume}{98}}, \bibinfo{pages}{207601}
  (\bibinfo{year}{2007}).

\bibitem{Stengel/Spaldin:2007}
\bibinfo{author}{Stengel, M.} \& \bibinfo{author}{Spaldin, N.~A.}
\newblock \bibinfo{title}{Ab-initio theory of metal-insulator interfaces in a
  finite electric field}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{75}},
  \bibinfo{pages}{205121} (\bibinfo{year}{2007}).

\bibitem{fixedd}
\bibinfo{author}{Stengel, M.}, \bibinfo{author}{Spaldin, N.~A.} \&
  \bibinfo{author}{Vanderbilt, D.}
\newblock \bibinfo{title}{Electric displacement as the fundamental variable in
  electronic-structure calculations}.
\newblock \emph{\bibinfo{journal}{arXiv:0806.0844}}  (\bibinfo{year}{2008}).

\bibitem{Bloechl:1994}
\bibinfo{author}{Bl{\"o}chl, P.~E.}
\newblock \bibinfo{title}{Projector augented-wave method}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{50}},
  \bibinfo{pages}{17953--17979} (\bibinfo{year}{1994}).

\bibitem{nature_2006}
\bibinfo{author}{Stengel, M.} \& \bibinfo{author}{Spaldin, N.~A.}
\newblock \bibinfo{title}{Origin of the dielectric dead layer in nanoscale
  capacitors}.
\newblock \emph{\bibinfo{journal}{Nature}} \textbf{\bibinfo{volume}{443}},
  \bibinfo{pages}{679} (\bibinfo{year}{2006}).

\bibitem{Kim_boundcharges}
\bibinfo{author}{Choi, W.~S.} \emph{et~al.}
\newblock \bibinfo{title}{Dielectric constants of {Ir}, {Ru}, {Pt}, and
  {IrO$_2$}: Contributions from bound charges}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{74}},
  \bibinfo{pages}{205117} (\bibinfo{year}{2006}).

\bibitem{Ghosez_etc}
\bibinfo{author}{Ghosez, P.}, \bibinfo{author}{Michenaud, J.-P.} \&
  \bibinfo{author}{Gonze, X.}
\newblock \bibinfo{title}{Dynamical atomic charges: The case of {ABO$_3$}
  compounds}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{58}},
  \bibinfo{pages}{6224--6239} (\bibinfo{year}{1998}).

\bibitem{Antons:2005}
\bibinfo{author}{Antons, A.}, \bibinfo{author}{Neaton, J.~B.},
  \bibinfo{author}{Rabe, K.~M.} \& \bibinfo{author}{Vanderbilt, D.}
\newblock \bibinfo{title}{Tunability of the dielectric response of epitaxially
  strained {SrTiO$_3$} from first-principles.}
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{71}},
  \bibinfo{pages}{024102} (\bibinfo{year}{2005}).

\bibitem{Giustino_ir}
\bibinfo{author}{Giustino, F.} \& \bibinfo{author}{Pasquarello, A.}
\newblock \bibinfo{title}{Infrared spectra at surfaces and interfaces from
  first principles: Evolution of the spectra across the {Si(100)-SiO$_2$}
  interface}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. Lett.}}
  \textbf{\bibinfo{volume}{95}}, \bibinfo{pages}{187402}
  (\bibinfo{year}{2005}).

\bibitem{Xifan_sl}
\bibinfo{author}{Wu, X.}, \bibinfo{author}{Stengel, M.}, \bibinfo{author}{Rabe,
  K.~M.} \& \bibinfo{author}{Vanderbilt, D.}
\newblock \bibinfo{title}{Predicting polarization and nonlinear dielectric
  response of arbitrary perovskite superlattice sequences}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. Lett.}}
  \textbf{\bibinfo{volume}{101}}, \bibinfo{pages}{087601}
  (\bibinfo{year}{2008}).

\bibitem{Ciacchi_PtO}
\bibinfo{author}{Seriani, N.}, \bibinfo{author}{Jin, Z.},
  \bibinfo{author}{Pompe, W.} \& \bibinfo{author}{{Colombi Ciacchi}, L.}
\newblock \bibinfo{title}{Density functional theory study of platinum oxides:
  From infinite crystals to nanoscopic particles}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{76}},
  \bibinfo{pages}{155421} (\bibinfo{year}{2007}).

\bibitem{Saad/Scott:2004}
\bibinfo{author}{Saad, M.~M.} \emph{et~al.}
\newblock \bibinfo{title}{Intrinsic dielectric response in ferroelectric
  nano-capacitors.}
\newblock \emph{\bibinfo{journal}{J.~Phys.: Condens. Matter}}
  \textbf{\bibinfo{volume}{16}}, \bibinfo{pages}{L451} (\bibinfo{year}{2004}).

\bibitem{explan-longit}
\bibinfo{note}{Strictly speaking, $D$ and $P$ here are the longitudinal
  components of the respective fields}.

\bibitem{Giustino/Pasquarello:2005}
\bibinfo{author}{Giustino, F.} \& \bibinfo{author}{Pasquarello, A.}
\newblock \bibinfo{title}{Theory of atomic-scale dielectric permittivity at
  insulator interfaces}.
\newblock \emph{\bibinfo{journal}{Phys. Rev. B}} \textbf{\bibinfo{volume}{71}},
  \bibinfo{pages}{144104} (\bibinfo{year}{2005}).

\end{thebibliography}
