PbTiO3is a prototypical ferroelectric material that exhibits a single structural phase transition (cubic to tetragonal): it is a soft mode driven, predominantly displacive, transition. In this paper, we study the behavior of PbTiO3at finite temperature by ab‐initio molecular dynamics simulations. In this approach classical mechanics is used to describe nuclear dynamics, while the interatomic potential is generated on the fly from the ground state of the electrons within density functional theory. Fluctuations of volume and shape of the simulation cell are included by means of Parrinello‐Rahman constant pressure scheme. Extensive convergence studies based on static calculations indicate that a 3×3×3 supercell containing 135 atoms, with a singlek‐point sampling, is sufficient to represent accurately the T = 0 energetics of this material. Although computationally demanding, ab‐initio molecular dynamics simulations for PbTiO3using a 3×3×3 cell are feasible with current computational methodologies. Here we report preliminary results of simulations that are both below and above the phase‐transition temperature. We discuss, in particular, how phonon softening occurs with temperature and how thermal expansion affects the results. © 2003 American Institute of Physics