To explore the energy flow-mechanism of a solution-chemical reaction on the basis of an energy-fluctuation analysis and the time evolution of various kinds of w Ž .x energies J. Phys. Chem. 48, 12506 1994 , a chemical reaction molecular dynamics simulation was carried out in the microcanonical ensemble for the proton-transfer reaction of formamidine in an aqueous solution. The energy ⌬ E required to surmount the reaction barrier was found to be supplied mainly from the potential energy of the solvent water rather than from the solvent kinetic energy. The ratio of the reactive energy flow from the solvent potential vs. the kinetic energy, ⌬Vr⌬ K, was 2.34 and was found to be in good agreement with the value of 1.96 predicted from the classical constant-volume B w heat capacity of water, C , via the Lebowitz᎐Percus᎐Verlet relation Phys. Rev. 153, 250 V Ž .x w 1967 . This finding confirmed the results of Wilson et al. J. Am. Chem. Soc. 113, 74 Ž .x 1991 ; namely, that the ratio should be determined only by the heat capacity of the solvent with no relation to the kinds of solute molecules, and in aqueous solution, the