The 129 local minima of the alanine tetrapeptide with relative energy < 10 kcal/mol were identified at the xB97X-D/6-311++ G(d, p) level of theory from initial structures generated by combining nine local minima of each residue. The CCSD(T), MP2, and dispersion-corrected DFT levels of theory with various basis sets were assessed for relative energies of the 24 representative conformations. The best performance was obtained at the double-hybrid DSD-PBEP86-D3BJ/def2-QZVP level of theory with RMSD = 0.12 kca l/mol against the CCSD(T)/CBS-limit energies. The xB97X-D/def2-QZVP and CAM-B3LYP-D3BJ/def2QZVP levels of theory can be an alternative level of theory with marginal deviations for conformational study of peptides. (C) 2018 Elsevier B.V. All rights reserved.