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AbĀ initioĀ selfāconsistentāfield (SCF) HartreeāFock and configuration interaction (CI) calculations have been carried out for H+Ā 2n+1Ā (n=1ā6) clusters using a tripleāzeta plus polarization basis set. Fully optimized structures and energies of H+Ā 11Ā and H+Ā 13Ā are presented. These structures can be thought as the addition of H2Ā molecules to a deformed H+9. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Mori. Nevertheless, our energy results on the biggest clusters suffer from the lack of size consistency of CI with single and double substitutions (CISD) calculations. Analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies and infrared intensities at the two levels of theory SCF (n=1ā6) and CISD (n=1ā4) both with tripleāzeta polarizationbasis sets. Of special interest are the new vibrational modes of H+Ā 11Ā and H+Ā 13, which have no counterpart in the H+9Ā cluster. Our predicted frequencies compare fairly well with the experimental results of Okumura, Yeh, and Lee. |