Assessment of CCSD(T), MP2, and DFT methods for the calculations of structures and interaction energies of the peptide backbone with water molecules

Highlights: • Structures and interaction energies of NMA and water molecules were explored. • ωB97X-D and M06-2X levels reproduced observed rotational constants of NMA and water hexamers. • Benchmark interaction energies were obtained by the CCSD(T)/CBS limit method. • The best performance was obtained using the double-hybrid DFTs against the benchmark energies. • ωB97X-D/def2-QZVP level can be an alternative for interaction energies of larger molecules. The CCSD(T), MP2, and DFT levels of theory with various basis sets are assessed for the structures and interaction energies of N-methylacetamide (NMA) and water molecules. The ωB97X-D/6–311++G(d,p) and ωB97X-D/6–31+G(2d,p) levels of theory seem to be adequate for optimization of the structures of NMA-water complexes. For seven NMA-water complexes, the best performances were obtained using the double-hybrid DSD-PBEP86-D3BJ and B2PLYP-D3BJ functionals with half-counterpoise corrections against the CCSD(T)/CBS-aTQ limit energies. However, the ωB97X-D/def2-QZVP//ωB97X-D/6–31+G(2d,p) level of theory can be an alternative method to calculate the interaction energies of the larger models.