  We have developed a shell model potential to describe PbTiO3 and
PbMg1/3Nb2/3O3 (PMN) by fitting to first-principles results. At zero pressure,
the model reproduces the temperature behavior of PbTiO3, but with a smaller
transition temperature than experimentally observed. We then fit a shell model
potential for the complex PMN based on the transferability of the interatomic
potentials. We find that even for ordered PMN, quenching the structure gives a
non-polar state, but with local polarization (off-center ions) indicative of
relaxor behavior.
