| Abstract
| - We report counterpoise (CP) corrected geometric optimizations for hydrated OH-, H3O+, and NH4+ usingvarious basis sets at the HF and MP2 levels. The CP corrections are calculated using two (CP2) and three(CP3) fragments. When three fragments are used, one is always H+. Symmetrical structures for H5O2+ andH3O2+ cannot be obtained with only two (necessarily nonequivalent) fragments. There are no significantdifferences in the CP-optimized interaction energies for these systems. Nevertheless, unlike the more traditionaltwo fragment procedure, the three fragment procedure allows convergence to symmetrical structures. Thislatter procedure is clearly preferable for systems such as OH-/H2O and H3O+/H2O while introducing nosignificant error when used for unambiguously unsymmetrical systems, such as NH4+/H2O.
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