Ground state of the (H2O)(2)(+) radical cation: DFT versus post-Hartree-Fock methods

Correlated calculations show the proton-transferred OH-H3O+ isomer to be th e ground-state structure of the (H2O)(2+) dimer ion, with the C-2h hemibond structure being ca. 8 kcal/mol less stable. Modern density functionals how ever favor the hemibond structure, overestimating the strength of the three -electron bond by ca. 17 kcal/ mel. The wrong prediction of the relative st ability of the two isomers is attributed to overestimation by the exchange functionals of the self-interaction part of the exchange energy in the hemi bond ion due to its delocalized electron hole. It is cautioned that this er roneous behavior of the density functionals for exchange, if unrecognized, may lead to wrong predictions for ground-state structures of systems with a three-electron bond.