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Unrestricted DFT methods are typically expected to describe the homolyticdissociation of nonpolar single bonds in neutral species with qualitativeaccuracy, due to the lack of significant delocalization error. We however findthat many widely used density functional approximations fail to describefeatures along the dissociation curve of the simple H$_2$ molecule. This is notan universal failure of DFT in the sense that many classic functionals like PBEand B3LYP give very reasonable results, as do some more modern methods likeMS2. However, some other widely used functionals like B97-D (empiricallyfitted) and TPSS (non-empirically constrained) predict qualitatively wrongstatic polarizabilities, force constants and some even introduce an artificialbarrier against association of independent H atoms to form H$_2$. Thepolarizability and force constant prediction failures appear to stem fromincomplete spin localization into individual H atoms beyond the Coulson-Fisherpoint, resulting in `fractionally bonded' species where the ionic contributionsto the Slater determinant are not completely eliminated, unlike the case ofunrestricted Hartree-Fock. These errors therefore appear to be a consequence ofpoor self-consistent density prediction by the problematic functional. The samereasons could potentially lead to spurious barriers towards H atom association,indirectly also leading to incorrect forces. These unphysicalities suggest thatthe use of problematic functionals is probably unwise in \textit{ab initio}dynamics calculations, especially if strong electrostatic interactions arepossible.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Well-behaved versus ill-behaved density functionals for single bond  dissociation: Separating success from disaster functional by functional for  stretched H$_2$