Using our recently implemented closed-shell coupled cluster singles and doubles (CCSD) model, the equilibrium structure and vibrational harmonic frequency of Nz, CO, HF and OH-have been determined. This set of diatomics was specially chosen so as to represent the range of bonding characteristics found in small molecules. The CCSD results are compared to analogous configuration interaction (CI) predictions and are found to be most similar to CISDQ or CISDTQ. That is, CI including all sin-gles+ doubles +quadruples (CISDQ) or CISDQ +a11 triple excitations (CISDTQ). The agreement of CCSD with CISDQXISDTQ is found to be much better for the singly bonded species with the agreement for neutral HF somewhat better than for anionic OH-. We arrive at two major conclusions. First, higher-order effects not present in the CCSD model are less negligible for multiply bonded species and, to a smaller degree, for anionic species as well. Secondly, even for the two types of molecules discussed above, the CCSD results are closer to the CISDQKISDTQ predictions than are CISD values. Since CCSD is less computationally time consuming than CISDQ/CISDTQ, we suggest that the CCSD model is one of the best methods available when highly accurate ab initio predictions for chemical species strongly dominated by a single-determinant wavefunction are desired.