BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS .6. 2ND-ROW-A(2) AND FIRST-ROW 2ND-ROW-AB DIATOMIC-MOLECULES

Benchmark calculations employing the correlation consistent basis sets of Dunning and co-workers are reported for the following diatomic spe cies: Al-2, Si-2, P-2, S-2, Cl-2, SiS, PS, PN, PO, and SO. Internally contracted multireference configuration interaction (CMRCI) calculatio ns (correlating valence electrons only) have been performed for each s pecies. For Cl-2, P-2, and PN, calculations have also been carried out using Moller-Plesset perturbation theory (MP2, MP3, MP4) and the sing les and doubles coupled-cluster method with and without perturbative t riples [CCSD, CCSD(T)]. Spectroscopic constants and dissociation energ ies are reported for the ground state of each species. In addition, th e low-lying excited states of Al-2 and Si-2 have been investigated. Es timated complete basis set (CBS) limits for the dissociation energies, D-e, and other spectroscopic constants are obtained from simple expon ential extrapolations of the computed quantities. At the CBS limit the root-mean-square (rms) error in D-e for the CMRCI calculations, the i ntrinsic error, on the ten species considered here is 3.9 kcal/mol; fo r r(e) the rms intrinsic error is 0.009 Angstrom, and for omega(e) it is 5.1 cm(-1).