The two dimensional potential energy surface for inversion of the methylene group and torsion of the methyl group in the ethyl radical has been calculated using B3LYP method and the aug-cc-pVQZ basis set. The potential was corrected for harmonic zero-point vibrational energies of remaining 13 motions and fitted to the mixed Fourier expansion. The rotation-inversion-torsion energies were calculated using a semi rigid model allowing relaxation of the molecular structure. The ab initio PES was modified to reproduce spectroscopic data. The effective barrier to internal rotation is as low as 2.2 cm(-1) comparing to the ab initio value of 18.9 cm(-1). (C) 2013 Elsevier B.V. All rights reserved.