Structure and energetics of hydrogen clusters. Structures of H+11 and H+13. Vibrational frequencies and infrared intensities of the H+2n+1 clusters (n=2–6)

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Abstract
Ab initio self‐consistent‐field (SCF) Hartree–Fock and configuration interaction (CI) calculations have been carried out for H+2n+1 (n=1–6) clusters using a triple‐zeta plus polarization basis set. Fully optimized structures and energies of H+11 and H+13 are presented. These structures can be thought as the addition of H2 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=1–6) and CISD (n=1–4) both with triple‐zeta polarization basis sets. Of special interest are the new vibrational modes of H+11 and H+13, which have no counterpart in the H+9 cluster. Our predicted frequencies compare fairly well with the experimental results of Okumura, Yeh, and Lee.