Evaluating Minnesota 2006 density functionals against some challenging problems in DFT

We tested the ability of Minnesota 2006 exchange-correlation functionals entailing M06-HF, M06-2X, M06 and M06-L in solving some challenging problems for density functional (DF) theory. It was found that these DFs cannot calculate the energy of a hydrogen atom well. Also, they show a smaller energy for stretched H₂⁺, which becomes much smaller by decreasing the %Hartree-Fock (HF) exact exchange. Unlike the case of H₂⁺, by increasing the %HF, the DFs overestimate by more the energy of H₂ molecule at infinity. For a coronene molecule, by enlarging %HF exchange, the LUMO and HOMO shift to higher and lower energies, respectively, widening the gap. We found that M06-HF and M06-2X are not suitable for electronic property calculations, and may not precisely represent the strong bonds in the dimers of metals. In summary, there is no systematic trend indicating that one Minnesota DF predicts acceptable distances and energies for rare gas dimers, and just M06-HF gives acceptable distances for all dimers. For the F atom and F₂ molecule, upon increasing the %HF, the electron affinity decreases and the results of M06-HF are much closer to experimental values. Compared to experimental results, the calculated adiabatic electron affinities are much more accurate than the vertical electron affinities.