Proper handling of hydrogen bonding using DFT remains a concern. Sherrill has reported a careful benchmark study using the potential energy curves for the six dimer combinations involving formic acid, formamide, and formamidine.1 Comparisons are made to the the CBS extrapolated limit CCSD(T) curve.
As anticipated, B3LYP and related functionals do a poor job. Interestingly, PBE and PBE0 provide very nice curves. The meta-GGA functionals like M05-2x and M06-2x and functionals with dispersion corrections (like ωB97X-D) provide very good potential energy curves. It is clear that intermediate and long-range correlation and dispersion must be accounted for when handling hydrogen bonded systems. Proper selection of the functional is critical.
References
(1) Thanthiriwatte, K. S.; Hohenstein, E. G.; Burns, L. A.; Sherrill, C. D., “Assessment of the Performance of DFT and DFT-D Methods for Describing Distance Dependence of Hydrogen-Bonded Interactions,” J. Chem. Theory Comput., 2011, 7, 88-96, DOI: 10.1021/ct100469b.
