We generated a reaction path for this reaction using IDPP and the NEB method (see Section 3 for details). In this reaction, the structure of butene is almost identical in the reactant and product; therefore, if the violation of the IDPP constraints were completely resolved, the structure of butene would be preserved along the path. However, the large initial violation due to atomic collisions along the initial path cannot be fully resolved by NEB optimization, and undesired molecular distortions and a bond breaking remain in the optimized path (Figures and). For example, the dihedral angle around the double bond of butene, which should remain zero, deviates by approximately ±30 • , and a part of the carbon chain becomes nearly linear. Additionally, the covalent bond formed between oxygen and carbon at both ends of the path is temporarily broken along the path.