Lattice dynamics for several ordered supercells with composition PbMg1/3Nb2/3O3(PMN) were calculated with first‐principles frozen phonon methods. Nominal symmetries of the supercells studied are reduced by lattice instabilities. Lattice modes corresponding to these instabilities, equilibrium ionic positions, and simulated infrared (IR) reflectance spectra are reported. © 2003 American Institute of Physics