A practical method is used to study chemical reactions in solution and in enzymes. In this method, referred to as the "classical trajectory mapping" method, the trajectories are calculated on a classical potential surface and the free energy profile of the chemical reaction is calculated by using this classical surface as a reference state for the actual ground state potential described by the semiempirical AM1 method. The speed of our method is shown to be faster than that of the conventional approach because our method does not require the time-consuming quantum mechanical calculations at each iteration. This method was applied to proton transfer reactions both in aqueous solution and in papain. The encouraging results indicate that our approach should be useful for studying chemical reactions in biological systems.