As described above, regression results were obtained using eq. 2 proposed by Xu et al. for complexes, eq. 3 proposed by Sverhebsky et al. for inorganic crystals of divalent cations, and eq. 4 proposed by us based on them. It is evident that eq. 4 exhibits the best regression performance for the 7,097 layered intercalation compounds calculated in this study. We then discuss the physical meaning of the optimal regression result of eq. 4.