The rotational potential function for the motion of of NH4+ in NH4PF6 has been derived from atom-atom potentials. The tunneling frequencies, librational frequencies, and rotational heat capacity have been computed for NH4+ and for ND4+ in NH4PF6. The tunneling frequencies of NH3D+ in NH4PF6 have also been calculated. (C) 2003 Elsevier B.V. All rights reserved.