Exploring chemistry through the source function for the electron and the electron spin densities

The Source Function, a chemical descriptor introduced by Bader and Gatti in 1998, represents a challenging tool to see the electron density from an unusual perspective. Namely, as caused, at any point in the space, by source contributions operating at all other points of space. Summing up the local sources over the atomic basins of a system, enable us to regard the electron density at any system’s location as determined by smaller or larger contributions from all the atoms or group of atoms of the system. Such decomposition of sources provides valuable chemical insight and it may be applied, on the same grounds, to theoretically or experimentally derived electron densities. Two recent Source Function developments, specifically its application to detect subtle electron delocalization effects and its extension to the electron spin density sources are reviewed through this chapter. An original application, as viewed through the eyes of the Source Function, then follows each illustrated development. Precisely: (a) the electron delocalization mechanisms in complex and non planar aromatic systems, like the homotropylium cation and the 1,6-methano[10]annulene, and (b) the spin density transferability properties in a series of n-alkyl radicals.