We study the dynamics of PF5, using a density functional potential energy surface. The full surface has 20 D-3h minima connected by 30 C-4v transition states. We use the theory of the molecular symmetry group to understand the tunnelling splittings of the vibrational levels. We estimate a value for the splitting by studying the motion between two equivalent minima. This splitting is very small for the ground vibrational state. Therefore we then study the vibrations of PF5 in one of its minima, using normal coordinates with anharmonic perturbation theory and variational theory. Our predicted fundamental frequencies agree well with observation. Finally we use the theory of the reaction path Hamiltonian to study the interaction of the vibrational modes with a large amplitude motion connecting two minima. (C) 2003 Elsevier Science B.V. All rights reserved.