The potential energy surface of the title reaction has been studied at
H-O...H-H distances between 1.0 and 2.1 Angstrom along a C-s reaction
path with a variety of density functionals, both local and nonlocal,
and compared to potential energy curves obtained at the Hartree-Fock,
MP2 and CCSD(T) levels. Whereas the latter curves are repulsive from 2
.1-1.4 Angstrom with energies above that of the reactants, nearly all
the DFT curves are either essentially flat or attractive with energies
below the reactants, giving an entirely incorrect picture of this imp
ortant radical reaction. Only the ACM functional - which includes part
of the exact Hartree-Fock exchange - gives a definite nonzero barrier
.