Equilibrium geometries, harmonic vibrational frequencies and the barrier heights of the internal rotation and inversion motion of methylamine (CH3NH2) conformers were calculated with the CCSD(T) method, in combination with the correlation consistent basis sets of Dunning. Valence and all electron correlation calculations were also carried out using the correlation consistent core and valence polarization triple-zeta basis set (cc-pCVTZ). Theoretical equilibrium geometries are in a general good agreement with available microwave and electron diffraction measurements. The best estimate results of the barrier heights for the internal rotation and inversion are, respectively, equal to 1.936 and 5.406 kcal/mol. (c) 2005 Elsevier B.V. All rights reserved.