Ref 18 shows that the conventional NEB method with IDPP leads to numerous undesired bond breakings, and, in contrast, an improved NEB method that carefully optimizes reaction paths by sequentially adding images provides an improved path that passes through a structure similar to Figure , where only the σ bond is temporarily broken. Unfortunately, the breaking of the σ bond observed in the improved path is also an undesired bond breaking caused by the vulnerability of IDPP. We will later demonstrate that the rotational isomerization of bianthracene can be achieved without breaking any covalent bonds.