  We have developed first-principles models, based on a general parametrization
of the full potential-energy surface, to investigate the lattice-dynamical
properties of perovskite oxides. We discuss the application of our method to
prototypic ferroelectric PbTiO3, showing that its properties are drastically
affected by a competition between structural instabilities. Indeed, we confirm
previous indications that the destructive interaction between the ferroelectric
and antiferrodistortive (involving rotations of the O6 octahedra) soft modes
shifts PbTiO3's Curie temperature by hundreds of degrees. Our theory provides
unique insight into this gigantic effect and its dynamical character.
