Ab initio calculations for small iodo clusters. Good performance of relativistic effective core potentials

The iodine clusters I₂(¹Σ_g ⁺), I₂(³Π_u), I₂ ^-(²Σ_u ⁺) and I₃ ^-(¹Σ_g ⁺), as well as excited states of I₂ ^-(²Σ_u ⁺) and I₃ ^-Σ_g ⁺), have been examined by means of different ab initio methods ranging from the SCF-HF to the QCISD(T) theoretical levels coupled with the relativistic effective core potential approach in a non-relativistic symmetry. The QCISD(T) method, with valence basis sets augmented by two sets of d polarization functions, was found to be reliable. The calculated equilibrium bond lengths, harmonic frequencies, force constants, bond dissociation energies, electronic transition energies and electron affinities are in good agreement with experimental data.