The Laplacian of electron density,  2 , is a function extensively employed in QTAIM theory. Notable positive and negative values of this function indicate that electron density is locally concentrated and depleted at the corresponding positions, respectively. In regions where covalent bonds are formed between elements in the first few periods, the value of this function is usually negative, so one can plot  2  isosurfaces or plane maps to visually identify the atoms that are covalently bonded. The usefulness of  2  in revealing covalent interactions is similar to that of ELF and LOL. However, since the  2  map often involves a very wide range of values, its graphical representation of covalent interaction areas is often not as ideal as ELF and LOL; additionally, it cannot effectively reveal covalent bonds between heavy atoms, such as the covalent bond in I2 molecules. Therefore,  2  is less commonly used today for visually studying covalent interactions. Interested readers are encouraged to refer to Bader's article, which specifically compares  2  and ELF, and more examples of  2  analysis can be found in Refs. .