The Source Function Descriptor as a Tool to Extract Chemical Information from Theoretical and Experimental Electron Densities

This chapter deals with the source function (SF) descriptor, originally put forth by Bader and Gatti back in 1998. After a brief review on how this descriptor is defined andwhat it physically represents, the various forms through which the SF may be analyzed are presented in some detail. The relationships between atomic SF contributions and chemical bond nature are analyzed in some prototypical cases, and the capability of the SF to neatly reveal p-electron conjugation directly from the electron distribution and independently fromanyMOscheme or decomposition is introduced. Applications of the SF to chemistry from the literature are reviewed and critically discussed, including the use of the SF to assess chemical transferability or to describe chemical bonding in challenging situations, like for instance the short-strong hydrogen bonds in p-conjugated frameworks or the metal-metal and metal-ligand interactions in the organometallic complexes. Comparison with the insight obtained from other bond topological descriptors is given, emphasizing the special role the SF has of being directly derivable fromexperimental electron density distributions and to so provide an ideal tool to compare experiment and theory. The robustness of the SF descriptor against changes in the models used to derive electron densities from theory of experiment is detailed. First results on using the SF to define an unambiguous full population analysis are outlined. The possible ways of further decomposing the atomic SF in chemically meaningful additive pieces, such as core and valence atomic contributions, are analyzed in view of their potential insight and degree of arbitrariness.