On the Interpretation of the Source Function

The chemical information present in the source function (SF) is analyzed in several case studies by decomposition into the relative contributions from the core and valence densities. Both experimental and quantum derived densities are examined, and for the latter case, the decomposition of the SF into contributions from the individual Kohn-Sham molecular orbitals is also investigated. For pairs of atoms A and B, the orbital decomposition of the SF at the A-B bond critical point SF(A-B)(bcp) is compared with that for the delocalization index delta(Omega(A),Omega(B)). For second and third period atoms, the valence density generally provides the determining contribution to the total SF, but for heavier elements such as transition metals, the core density plays an increasingly important role. Moreover, when the reference point is close to the nodal plane of all orbital, this orbital makes a low to negligible contribution to the SF, which has clear implications for the interpretation of pi-interactions. This leads us to recommend caution in associating some chemical concepts with features of the SF, especially for heavier elements.