We compared the protein-ligand binding free energies (flG) obtained by the explicit water model, the MM-G8/SA (molecular-mechanics generalized 80m surface area) model, and the docking scoring function. The free energies by the explicit water model and the MM-G8/SA model were calculated by the previously developed Smooth Reaction Path Generation (SRPG) method. In the SRPG method, a smooth reaction path was generated by linking two coordinates, one a bound state and the other an unbound state. The free energy surface along the path was calculated by a molecular dynamics (MD) simulation, and the binding free energy was estimated from the free energy surface. We applied these methods to the streptavidin-and-biotin system. The flG value by the explicit water model was close to the experimental value. The flG value by the MM-G8/SA model was overestimated and that by the scoring function was underestimated. The free energy surface by the explicit water model was close to that by the G8/SA model around the bound state (distances of < 6 A), but the discrepancy appears at distances of> 6 A. Thus, the difference in long-range Coulomb interaction should cause the error in flG. The scoring function cannot take into account the entropy change of the protein. Thus, the error of flG could depend on the target protein.