  The electronic ground state of a periodic crystalline solid is usually
described in terms of extended Bloch orbitals; localized Wannier functions can
alternatively be used. These two representations are connected by families of
unitary transformations, carrying a large degree of arbitrariness. We have
developed a localization algorithm that allows one to iteratively transform the
extended Bloch orbitals of a first-principles calculation into a unique set of
maximally localized Wannier functions. We apply this formalism here to the case
of cubic BaTiO3. The purpose is twofold. First, a localized-orbital picture
allows a meaningful band-by-band decomposition of the whole Bloch band complex.
In perovskites, these Wannier functions are centered on the atomic sites and
display clearly a s, p, d, or hybrid character. Second, since the centers of
the Wannier functions map the polarization field onto localized point charges,
the ground state dielectric properties become readily available. We study the
Born effective charges of the paraelectric phase of BaTiO3. We are able to
identify not only the contributions that come from a given group of bands, but
also the individual contributions from the ``atomic'' Wannier functions that
comprise each of these groups.
