An extensive quantum mechanical study of a water dimer suggests that the introduction of a diffuse function into the basis set, which Ž . significantly reduces the basis set superposition error BSSE in the hydrogen bonding energy calculation, is the key to better calculations of the potential energy surfaces of carbohydrates. This article examines the potential energy Ž . surfaces of selected D-aldo-and D-ketohexoses a total of 82 conformers by Ž . Ž . quantum mechanics QM and molecular mechanics MM methods. In contrast Ž . to the results with a smaller basis set B3LYPr6-31G** 5d , we found at the Ž Ž . . higher level calculation B3LYPr6-311qqG 2d,2p rrB3LYPr6-31G** 5d that, in most cases, the furanose forms are less stable than the pyranose forms. These discrepancies are mainly due to the fact that intramolecular hydrogen bonding energies are overestimated in the lower level calculations. The higher level QM calculations of the potential energy surfaces of D-aldo-and D-ketohexoses now are more comparable to the MM3 results.