First, it can be observed that the  val isosurfaces intuitively show the distribution of electrons involved in bonding. Based on the size of the isosurfaces in the bonding areas, the relative strength of the C-C bonds can be easily inferred. For example, in structure (a), the  val between atoms of the typical double bond (C5-C6) and the approximate double bonds (C1-C2 and C3-C4) is much higher than that between C2-C3, which is close to a single bond. As the reaction progresses, it is clearly seen from the figure that the  val on the C1-C2 and C5-C6 bonds gradually decreases, while the  val on the C2-C3 bond gradually increases. At the transition state structure, the isosurfaces of all C-C bonds are almost the same, indicating that the strengths of these bonds are nearly indistinguishable. In the product structure (d), the  val distribution on the C2-C3 bond is roughly equivalent to that of the C5-C6 bond in the reactant structure, reflecting that the C2-C3 bond has finally become a double bond.