The M06-L density functional has been assessed for its ability to predict the correct structure of a Tyr-Gly conformer for which MP2/6-31+G(d) predicts a folded 'closed book' conformer, whereas a more open conformation is predicted by B3LYP/6-31+G(d). Potential energy profiles were determined by computing the energy for geometries optimised at various fixed values of a distance that controls the degree of foldedness of the structure. The M06-L/6-31G(d) and M06-L/6-31+G(d) methods yield excellent agreement with the reference df-LCCSD(T0)/aug-cc-pVTZ profile. Thus, M06-L manifests itself as a very promising method to investigate the potential energy surface of small peptides containing aromatic residues. (C) 2009 Elsevier B. V. All rights reserved.