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We report computer simulation results of free energies of proton-disordered hexagonal ice and liquid water at atmospheric pressure with the 4-site transferable intermolecular potential model of water. A new reference system is used to calculate the free energy of the ice phase. The melting point of proton-disordered hexagonal ice at atmospheric pressure is found to be Tm=238 (±7) K. This result is consistent with a previous estimation, 230K< TmK, from molecular dynamic simulation of the surface melting of the proton-disordered hexagonal ice [G.-J. Kroes, Surf. Sci. 275, 365 (1992)].