The vibrational properties of a crystal determine a wide range of macroscopic behaviour: thermal properties such as the specific heat, transport properties like resistivity and velocity of sound, and the interaction with radiation, for example in infra-red absorption and Raman scattering. The optical effects are important tools for following the progress of phase transitions. Very low frequency modes are implicated in structural phase transitions, and the presence of unstable modes in a proposed structure can suggest alternative, more stable structures. This has been particularly exploited in perovskite structures which display a wide range of phase transitions (10; 11; 12; 9). Ab initio calculations of phonon frequencies and eigenvectors are thus very useful not only in predicting macroscopic properties, but also in tracing mechanisms of phase transitions. Phonon eigenvectors are not experimentally accessible, beyond determination of the symmetry and polarisation, so theoretical work can complement experimental investigation, for example by following the changing nature of modes under pressure (13; 14).