In the present study, the performance of six popular density functionals
(B3LYP, PBE0, BLYP, BP86, PBE, SVWN) for the description of the autoionization
process in the water octamer was studied. As a benchmark, MP2
energies with complete basis sets limit extrapolation and CCSD(T) correction
were used. At this level the autoionized structure lies 28.5 kcal.mol.1
above the neutral water octamer. Accounting for zero point energy lowers
this value by 3.0 kcal.mol.1. The transition state of the proton transfer
reaction, lying only 0.7 kcal.mol.1 above the energy of the ionized system,
was identified at the MP2/aug-cc-pVDZ level of theory. Different density
functionals describe the reactant and product with varying accuracy, while
they all fail to characterize the transition state. We find improved results
with hybrid functionals compared to the gradient-corrected ones. In particular,
B3LYP describes the reaction energetics within 2.5 kcal.mol.1 of the
benchmark value. Therefore, this functional is suggested to be preferably
used both for Car-Parinello molecular dynamics and QM/MM simulations of
autoionization of water.