Ab initio calculation of proton barrier and binding energy of the (H2O)OH- complex

The very low barrier (of the order of I U mol to proton transfer in the (H2O)OH- complex is accurately computed by the coupled-cluster method including single and double excitations plus perturbative corrections for triple excitations [CCSD(T)]. By virtue of the coupled-cluster R12 method [CCSD(T)-R12], which employs explicitly correlated wave functions, results are obtained close to the limit of a complete basis set of atomic orbitals (AOs). The basis-set superposition error is avoided by applying the full function counterpoise approach to the three fragments OH-, H+, and OH-. The most accurate computed value for the proton barrier amounts to delta = 0.9 +/- 0.3 kJ mol(-1), which is 4-5 times this value below the lowest vibrational energy level. The three lowest vibrational energy levels are calculated from a one-dimensional potential energy curve describing the position of the bonding proton between the two OH- fragments. The electronic binding energy with respect to dissociation into H2O and OH- is estimated to D-c = 109 +/- 10 kJ mol(-1) at the level of explicitly correlated CCSD(T)-R12 theory. (C) 2002 Elsevier Science B.V. All rights reserved.