A basic concept in thermodynamics states that, if an isolated system is left standing, it comes eventually to a final state which does not change. This final state is called the thermal equilibrium state. If vacancies and self-interstitials coexist, it is sure that they recombine with each other. Greatest importance is given to the basic concept in the thermodynamics mentioned above. Therefore, regardless of whether or not the thermal generation of the Frenkel pair is energetically possible, the Frenkel pair should be generated thermally as a reverse reaction of the recombination, to get the thermal equilibrium state.