Benchmark calculations with correlated molecular wave functions. VI. Second row A₂ and first row/second row AB diatomic molecules

Benchmark calculations employing the correlation consistent basis sets of Dunning and co-workers are reported for the following diatomic species: Al ₂, Si₂, P₂, S₂, Cl₂, SiS, PS, PN, PO, and SO. Internally contracted multireference configuration interaction (CMRCI) calculations (correlating valence electrons only) have been performed for each species. For Cl₂, P₂, and PN, calculations have also been carried out using Møller-Plesset perturbation theory (MP2, MP3, MP4) and the singles and doubles coupled-cluster method with and without perturbative triples [CCSD, CCSD(T]. Spectroscopic constants and dissociation energies are reported for the ground state of each species. In addition, the low-lying excited states of Al₂ and Si₂ have been investigated. Estimated complete basis set (CBS) limits for the dissociation energies, D_e, and other spectroscopic constants are obtained from simple exponential extrapolations of the computed quantities. At the CBS limit the root-mean-square (rms) error in D_e for the CMRCI calculations, the intrinsic error, on the ten species considered here is 3.9 kcal/mol; for r_e the rms intrinsic error is 0.009 Å, and for ω_e it is 5.1 cm^-1.