shows ethane, ethylene, and acetylene. It can be seen that the ELF isosurfaces of the C-C single bond, double bond, and triple bond are all distributed in the bonding region, but their shapes differ significantly. The C-C single bond has an elliptical shape symmetrical around the bond axis, the double bond has a cone shape extending in the direction of π-electron distribution, and the triple bond has an oblate shape with a concave center, symmetrical around the bond axis. Since the shape of the ELF isosurface is highly sensitive to bond multiplicity, ELF can be used to visually detect bonding types. The multiple bonds between transition metals can also be displayed using ELF. For example, Fig. (b) shows the Mo2 molecule, which formally has a sextuple bond. The ELF isosurface corresponding to this bond is a very large ring, as both the π-and δ-bonds between the two atoms are doubly degenerate. Fig. shows cyclopropane, where it can be seen that the center of the ELF isosurface corresponding to the C-C σ-bond is not exactly on the bond axis but deviates outward from the system. This demonstrates that ELF can faithfully reveal the offset of bonding electrons caused by ring strain. where valence electrons can no longer freely delocalize over the system, and some localized electrons appear in the interstitial regions, which are also known as pseudoanions. It can be seen that ELF can intuitively reveal main distribution areas of these localized electrons. [C60AuC60]⁺ system reported by Goulart and coworkers. The high localization areas indicated by the white arrows clearly demonstrate the existence of the covalent interaction between C60 and Au. Fig. ) is a map from our study on the effect of external electric fields (EEF) on various properties of cyclo [18]carbon. It can be seen that under the influence of a very strong EEF of 0.029 a.u., not only does the structure of cyclo [18]carbon change from a circle to an ellipse, but some of its electrons are almost detached from the system. Additionally, from the electron localization characteristics of the C-C bonds shown by ELF, it is evident that the strength of the C-C covalent bonds in this system alternates significantly. Fig. 19(d) shows potassium hydroxide. It is well known that there is an ionic bond between K and OH⁻. From its ELF map, it can be seen that the electron localization in the bonding region is quite low. This is a typical feature of noncovalent interactions.