  We carry out first-principles calculations for CaTiO$_3$/BaTiO$_3$
superlattices with epitaxial strain corresponding to growth on a SrTiO$_3$
substrate, and consider octahedral rotations as well as ferroelectric
distortions. The calculations are done as a function of electric displacement
field, and both a macroscopic and a local electrostatic analysis are carried
out. We find that strong octahedral rotations occur for TiO$_6$ octahedra
sandwiched between CaO layers on both sides, but are strongly suppressed if
either neighboring layer is a BaO layer. Due to the resulting enhancement of
the ferroelectric instability in the BaO-neighboring octahedra, we find that
overall the ferroelectric instability of the superlattice is enhanced by the
interface. Thus, short-period superlattices in this system have a larger
ferroelectric polarization than longer-period ones of the same average
composition, contrary to the expected trend.
