Molecular dynamics simulation is used to study how the solute-solvent potential affects the classical free energy surfaces of the reactant and product states in the N-H...O=C proton transfer process in an aqueous medium when the atomic charges that describe the electrostatic contribution of the potential are modified. The strong dependence of the thermodynamic properties (DeltaG, DeltaG(r), and DeltaG(#) free energies) associated with the proton transfer on the charge definition employed led us to look for a new procedure to calculate the atomic charges which is better suited to the method used in the determination of the free energy curves via the solute-solvent interaction energy. (c) 2002 Elsevier Science B.V. All rights reserved.