Assessment of Density Functionals for Intramolecular Dispersion-Rich Interactions

A range of density functional theory methods, including conventional hybrid and meta-hybrid functionals, a double-hybrid functional, and DFT-D (DFT augmented with an empirical dispersion term) were assessed for their ability to describe the three minima along the ϕGly rotational profile of one particular Tyr-Gly conformer. Previous work had shown that these minima are sensitive to intramolecular dispersion and basis set superposition error, the latter rendering MP2 calculations with small to medium-sized basis sets unsuitable for describing this molecule. Energy profiles for variation of the ϕGly torsion angle were compared to an estimated CCSD(T)/CBS reference profile. The hybrid functionals and the meta-hybrid PWB6K failed to predict all three minima; the meta-hybrid functionals M05-2X and M06-2X and the nonhybrid meta functional M06-L as well as the double-hybrid mPW2-PLYP and the B3LYP-D method did find all three minima but underestimated the relative stability of the two with rotated C-terminus. The best performance was delivered by the most elaborate density functional theory model employed: mPW2-PLYP-D. Only M06-2X and mPW2-PLYP-D predicted the correct order of stability of the three minima.