  The ferroelectric trends of rutile (TiO$_2$) in bulk and at the (110) surface
are investigated by means of ab initio density functional theory. We discuss
the underlying mechanism of the incipient ferroelectric behavior of rutile in
terms of Born effective charges, which we decompose in individual contributions
by means of maximally localized Wannier functions. We show that a ferroelectric
phase can be stabilized for a variety of different lattice distortions which
all enlarge the shortest Ti-O distance, even if the longer apical Ti-O bond is
simultaneously shortened. At the (110) surface, the ferroelectric trends are
modified compared to the bulk, but nevertheless ferroelectric phases with large
polarization even in the topmost surface layer can be stabilized by uniaxial
strain.
