Water Dimer Cation: Density Functional Theory vs Ab Initio Theory.

By using density functional theory (DFT) and high-level ab initio theory, the structure, interaction energy, electronic property, and IR spectra of the water dimer cation [(H2O)2(+)] are investigated. Two previously reported structures of the water dimer cation [disproportionated ionic (Ion) structure and hydrazine-like (OO) structure] are compared. For the complete basis set (CBS) limit of coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)], the Ion structure is much more stable (by 11.7 kcal/mol). This indicates that the ionization of water clusters produce the hydronium cation moiety (H3O(+)) and the hydroxy radical. The transition barrier for the interconversion from the Ion/OO structure is ∼15/∼9 kcal/mol. It is interesting to note that the calculation results of the water dimer cation vary seriously depending on calculation methods. Möller-Pleset second-order perturbation (MP2) theory gives reasonable relative energies in favor of the Ion structure but reports unreasonable frequencies for the OO structure. On the other hand, most DFT calculations with various functionals overstabilize the OO structure. However, the DFT results with MPW1K and BH&HLYP functionals are very close to the CCSD(T)/CBS results. Thus, as for the validity test of the DFT functionals for ionized molecular systems, the energy comparison of two water dimer cation structures would be a very important criterion.