  Due to the severe self-interaction errors associated with some density
functional approximations, conventional density functionals often fail to
dissociate the hemibonded structure of water dimer radical cation (H2O)2+ into
the correct fragments: H2O and H2O+. Consequently, the binding energy of the
hemibonded structure (H2O)2+ is not well-defined. For a comprehensive
comparison of different functionals for this system, we propose three criteria:
(i) The binding energies, (ii) the relative energies between the conformers of
the water dimer radical cation, and (iii) the dissociation curves predicted by
different functionals. The long-range corrected (LC) double-hybrid functional,
omegaB97X-2(LP) [J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2009, 131,
174105.], is shown to perform reasonably well based on these three criteria.
Reasons that LC hybrid functionals generally work better than conventional
density functionals for hemibonded systems are also explained in this work.
