We compare several standard polarized double-zeta basis sets for use in full configuration interaction benchmark computations. The 6-31G(**), DZP, cc-pVDZ, and Widmark-Malmqvist-Roos atomic natural orbital (ANO) basis sets are assessed on the basis of their ability to provide accurate full configuration interaction spectroscopic constants for several small molecules. Even though highly correlated methods work best with larger basis sets, predicted spectroscopic constants are in good agreement with experiment; bond lengths and harmonic vibrational frequencies have average absolute errors no larger than 0.017 Angstrom and 1.6%, respectively, for all but the ANO basis. For the molecules considered, 6-31G(**) gives the smallest average errors, while the ANO basis set gives the largest. The use of variationally optimized basis sets and natural orbitals are also explored for improved benchmarking. Although optimized basis sets do not always improve predictions of molecular properties, taking a DZP-sized subset of the natural orbitals from a singles and doubles configuration interaction computation in a larger basis significantly improves results. (C) 2003 American Institute of Physics.