Use of atom based truncation schemes in thermodynamic integration leads to hydration free energies of water that depend strongly on the cut-off radius. By contrast, Ewald summation, a reaction field technique, and group based truncation of the potential give the same result (6.5 +/- 0.1 kcal/mole), independently of the cut-off radius and system size. (C) 2000 American Institute of Physics. [S0021-9606(00)51916-3].