Ab initio calculations have been carried out on model compounds for the pyranose halides 2-fluoro- and 2-chlorotetrahydropyran with either an axially or an equatorially oriented halogen atom. Energy minimization has been carried out at the STO-3G, 3-21G, 6-31G, 6-31G*, 6-31+G*, and MP2/6-31G* levels. The optimized geometries were used to calculate the energy difference between the axial and equatorial conformers with STO-3G, 3-21G, 4-31c, 6-31G, 6-31G*, 6-31G**, 6-31+G*, 6-311G*, 6-31SG**, 6-311++G**, and MP2/ 6-31G* basis sets. Large differences in C-Hal bond lengths and O-C-Hal bond angles were found between the axial and equatorial conformers. After including the zero-point energy, thermal energy, entropy, and MP3 electron correlation corrections to energy differences calculated at 6-311++G**//6-31+G* basis set, these calculations favored the axial conformers by 2.4 and 2.5 kcal/mol. Solvent effects considerably reduce this energy difference; in the extreme case, in water, values of 0.5 and 1.5 kcal/mol were obtained for fluoro and chloro derivatives, respectively. The magnitude of the anomeric effect depends on the solvent and was estimated to be in the range 0.9-2.8 kcal/mol for the fluoro and 2.3-3.1 kcal/mol for the chloro forms. On the basis of these results, we suggest that the 6-31+G*//6-31G* procedure is suitable for calculations of the geometry and the conformational energies of carbohydrate molecules. The calculated energies and geometries provide additional data which should prove useful in the reparametrization of existing force fields to better reproduce the behavior of C-O-Hal systems.