CCSDT CALCULATIONS OF MOLECULAR EQUILIBRIUM GEOMETRIES

CCSDT equilibrium geometries of CO, CH2, F-2, HF, H2O and N-2 have bee n calculated using the correlation-consistent cc-pVXZ basis sets. Simi lar calculations have been performed for SCF, CCSD and CCSD(T). In gen eral, bond lengths decrease when improving the basis set and increase when improving the N-electron treatment. CCSD(T) provides an excellent approximation to CCSDT for bond lengths as the largest difference bet ween CCSDT and CCSD(T) is 0.06 pm. At the CCSDT/cc-pVQZ level, basis s et deficiencies, neglect of higher-order excitations, and incomplete t reatment of core-correlation all give rise to errors of a few tenths o f a pm, but to a large extent, these errors cancel. The CCSDT/cc-pVQZ bond lengths deviate on average only by 0.11 pm from experiment. (C) 1 997 Elsevier Science B.V.