The cytosine-(H2O)(14) and adenine-(H2O)(16) Complexes were studied using B3LYP/6-31+G(d), LMP2/aug-cc-pVnZ (n = D, T) and the new semiempirical method PM6. For each complex, two optimized structures were examined: one with water molecules distributed around the central base and one with a clustering of water molecules. The clustered structures were found to be energetically favored at all levels of theory employed (except at the B3LYP level with inclusion of zero-point energy correction, where the two cytosine(H2O)(14) structures are nearly iso-energetic). (C) 2007 Elsevier B.V. All rights reserved.