Challenges in predicting rxnG in solution: Hydronium, hydroxide, and water autoionization

Standard non-semiempirical continuum-dielectric orbital-based methods horribly overpredict, by 26-50 kcal mol(-1), the Gibbs energy for the water autoionization reaction 2H(2)O((l)) H3O(aq)++OH(aq)-. Here, we demonstrate these errors, fully investigate the reasons for these errors, and show that the use of 4 explicit solvent within the continuum (the semicontinuum, cluster-continuum, or hybrid technique) can reduce the error of a standard continuum model from 50 to 2 kcal mol(-1). Results from pure cluster, pure continuum (several versions including semiempirical ones), and semicontinuum modeling are each presented and discussed. We recommend use of 3 waters around hydronium and 4 waters around hydroxide with standard continua whenever these ions are involved in reaction. To the possible surprise of some, time-consuming molecular-dynamics simulations are not needed to reproduce this problematic energy.