Insufficient description of dispersion in B3LYP and large basis set superposition errors in MP2 calculations can hide peptide conformers

B3LYP/6-31+G(d) and MP2/6-31+G(d) calculations predict markedly different structures for one Tyr-Gly conformer. Calculation of the energy profile for rotation around the glycine C-alpha-N bond reveals one minimum in the B3LYP profile (phi(gly) = 180 degrees) and two in the MP2 profile (similar to 75 degrees and 280 degrees). Large intramolecular BSSE values are responsible for masking the 180 degrees-minimum in the MP2 profile: approximate elimination of BSSE in the MP2 calculations - by (1) correction using BSSE values from complexes of phenol and N-formylglycine, (2) the application of local MP2, or (3) employing large basis sets (aug-cc-pVTZ/QZ) and density fitting - yields an unambiguous triple-well potential. (C) 2007 Elsevier B.V. All rights reserved.