  Most d^0 transition metal (TM) oxides exhibit anomalously large Born
dynamical charges associated with off-centering or motion of atoms along the
TM-O chains. To understand their chemical origin, we introduce "Wannier Orbital
Overlap Population" (WOOP) and "Wannier ond Orbital Position Population" (WOPP)
based on the Wannier function based description of electronic structure
obtained within first-principles density functional theory. We apply these
concepts in a precise analysis of anomalous dynamical charges in PbTiO_3,
BaTiO_3 and BaZrO_3 in the cubic perovskite structure. Determining
contributions of different atomic orbitals to the dynamical charge and their
break-up into local polarizability, charge transfer and covalency, we find that
p orbitals of oxygen perpendicular to the -TM-O- chain contribute most
prominently to the anomalous charge, by facilitating a transfer of tiny
electronic charge through one unit cell from one TM atom to the next. Our
results explain why the corner-shared linkage of TMO_6 octahedra, as in the
perovskite structure, is ideal for large dynamical charges and hence for
ferroelectricity.
