A coupled motion of the skeletal inversion and methyl torsion in trimethylamine is investigated theoretically at various levels up to MP4(SDQ)/aug-cc-pVTZ. Among possible structures, the pyramidal C-3v is the only minimum energy structure, and the planar C-s is the transition state structure of the nitrogen inversion. Our best estimate of the barrier height (without zero-point energy) is 3290 cm(-1). The quasi-classical direct ab initio MD using HF/6-31G* starting from the pyramidal C-3v structure indicates that the coupled methyl torsional motion is the primary factor for the nitrogen inversion of trimethylamine. (c) 2005 Elsevier B.V. All rights reserved.