Interestingly, this reaction reveals a common limitation of FB-ENM and IDPP: When an atom forms one bond only in the reactant and another only in the product, both methods cannot describe the correlation between the breaking and formation of these bonds. In this reaction, the exchanged hydrogen atoms become isolated from both ammonia and formaldehyde along the FB-ENM path or IDPP path (Figures and). However, the TSs shown in Figures and suggest that the formation of one bond occurs almost simultaneously with the breaking of the other bond. In fact, the anomalous isolation of hydrogens results in significantly higher energies than the TS energies (Figure ). Describing the correlation between the formation and breaking of two bonds emerging from an atom remains a future challenge.