Molecular properties of computationally difficult diatomic systems: O-2, F-2(-), Cl-2(-), SiF, SiCl, ClO and MgO

For seven difficult diatomic molecules, O-2, F-2(-), Cl-2(-), SiF, SiCl, Cl O and MgO, spectroscopic constants, R-e, B-e, omega (e), omega (e)chi (e), and D-0, have been calculated by two computational methods, MR-ACPF and CCS D(T). The sequence of aug-cc-pVnZ+ basis sets has been used, and for O-2, s ubsequent extrapolation of the total energies has been performed. In partic ular, new bond lengths for F-2(-) and Cl-2(-) are presented, and the establ ished experimental dissociation energies for SiF and SiCl are corrected. Th e dissociation energy of the O-2 molecule has been calculated by different methods, overcoming the deficiencies of the aug-cc-pVnZ basis sets, at an a ccuracy of at least 0.3 kcalmol(-1).