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 Cα–N bond reveals one minimum in the B3LYP profile (ϕgly = 180°) and two in the MP2 profile (∼75° and 280°). Large intramolecular BSSE values are responsible for masking the 180°-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.