Using first-principles calculations, we investigate the lattice dynami
cs of cubic and rhombohedral BaTiO3 and rye discuss the origin of the
structural instability of this ferroelectric material. First, we repor
t results on the Born effective charges and the dielectric tensor and
we emphasize the important reduction of these quantities in the ferroe
lectric phase, Then, the phonon frequencies at the Gamma point are cal
culated. We point out the similarity of theoretical eigenvectors in th
e cubic and rhombohedral phases. We examine the interaction of the vib
ration modes with the electric field and in particular the giant LO-TO
splitting of the ferroelectric mode. Finally, separating the dipole-d
ipole interaction from the remaining short-range forces, we quantify t
he balance of forces leading to an unstable phonon in the cubic phase
and we demonstrate its sensitivity to tiny effective charge changes. W
ithin our decomposition, the stabilization of the unstable mode in the
rhombohedral phase is produced by a reduction of the Born effective c
harges, while its stabilization under isotropic pressure is associated
with a modification of the short-range forces.