The bond order, especially the one proposed by Mayer, 21 has been widely used to characterize covalent bonds in molecules. The corresponding bond energy, which can be obtained using various schemes to decompose the total energy of a molecule, can in principle also be employed to assess the strength change of chemical bonds in a reaction. Separately, the bond order can also be gauged via the local vibrational analysis , and a chemical reaction can be analyzed using the related unified reaction valley approach (URVA). Topological analyses of electron density, density gradient, and wavefunction based quantities [such as the electron localization function (ELF), non-covalent interactions (NCI) analysis, and dynamic Voronoi metropolis sampling (DVMS) ] have been proposed by several research groups. These tools originated from the idea of partitioning the molecular space into different regions, as in the aforementioned Bader's Atoms-in-Molecule (AIM) theory. The ELF function from Becke and Edgecombe, for instance, can be used to divide the electron density into the core basins connecting two nuclei and valence basins representing non-bonding regions and lone pairs and to study the bond evolution in chemical reactions. Similarly, the NCI plot can be used to identity the key non-covalent interaction regions in molecules at the equilibrium geometry and transition states. More recently, the DVMS method utilized a metropolis walk in the electron configuration space to acquire wavefunction tiles -signed regions of the wavefunction -that can be followed to visualize the motion of the electrons along a reaction pathway.