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Reference quality conformational energies have been obtained for the 52 unique conformers of melatonin by means of explicitly correlated ab initio methods as well as the ccCA composite method. These data have then been used to evaluate more approximate methods, including a variety of density functionals both on their own and paired with various empirical dispersion corrections. Owing to the presence of internal contacts of the C-H···O and C-H···N variety, basis set convergence is much slower than for alkane conformers, for example, and basis sets of aug-cc-pVQZ or def2-QZVP quality seem to be required to obtain firm estimates of the basis set limit. Not just HF, but also many DFT functionals, will transpose the two lowest conformers unless empirical dispersion corrections are added. Somewhat surprisingly, many DFT functionals reproduce the reference data to fairly high accuracy when combined with the D3BJ empirical dispersion correction or the "nonlocal" Vydrov-Van Voorhis dispersion model. The two best performers including dispersion corrections are the double hybrids DSD-PBEP86-D3BJ and B2GP-PLYP-D; if no such correction is permitted, then M06-2X puts in the best performance. Of lower-cost ab initio-like models, MP2.5 yields the best performance, followed by SCS-MP2.

The Melatonin Conformer Space: Benchmark and Assessment of Wave Function and DFT Methods for a Paradigmatic Biological and Pharmacological Molecule