BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS .12. CORE CORRELATION-EFFECTS ON THE HOMONUCLEAR DIATOMIC-MOLECULES B-2-F-2

Using systematic sequences of the newly developed correlation consiste nt core-valence basis sets from cc-pCVDZ through cc-pCV6Z, the spectro scopic constants of the homonuclear diatomic molecules containing firs t row atoms, B-F, are calculated both with and without inclusion of 1s correlation. Internally contracted multireference configuration inter action (IC-MRCI) and singles and doubles coupled cluster (CCSD) theory with a perturbational estimate of connected triple excitations, CCSD( T), have been investigated. By exploiting the convergence of the corre lation consistent basis sets, complete basis set (CBS) limits have bee n estimated for total energies, dissociation energies, equilibrium geo metries, and harmonic frequencies, Based on the estimated CBS limits t he effects of 1s correlation on D-e (kcal/mol), r(e) (Angstrom), and o mega(e) (cm(-1)) are: +1.1, -0.0070, +10 for B-2; +1.5, -0.0040, +13 f or C-2; +0.9, -0.0020, +9 for N-2; +0.3, -0.0020, +6 for O-2; and -0.1 , -0.0015, +1 for F-2.