Making use of CEPA-1, CCSD and CCSD(T) calculations with relatively large basis sets and taking the major anharmonicity effects into account, predictions are made for the following 12 molecules which are of interest to interstellar cloud chemistry: HC;

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Dedicated to Dr Gerhard Herzberg on the occasion of his 90th birthday Recent large-scale ab initio calculations for small negative molecular ions are reviewed. Accurate equilibrium geometries are established for several species like NH; , HCC -, NO;, CH2N -, C; and C , . Predictions are made for various spectroscopic properties like vibrational frequencies, rotational constants and infrared intensities. The effects of a shallow energy minimum in the T-shaped configuration on the rovibrational term energies of HCC -are investigated. The calculated vibrational structures of the photoelectron spectra of SiH; and CH2N -are in very good agreement with the experiment. The present calculations support t$e assig_nment of an absorption observed at 608nm in a neon matrix to the C2n, t X'I'I, transition of C, . Electron affinities are obtained with an accuracy of 0.05eV or better. Spectroscopic properties of intermediates in simple S,2 reactions are calculated.

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