CHEMICAL INFORMATION FROM THE SOURCE FUNCTION

C. Gatti,a F. Cargnoni,a and L. Bertinia

aCNR-ISTM, Istituto di Scienze e Tecnologie Molecolari, via Golgi 19, 20133 Milano, Italy (c.gatti@istm.cnr.it)

Recently it was shown [1] that one may view the electron density at any point r within a molecule to consist of contributions from a source G(r , ) operating at all other points . 

By evaluating the source over regions bounded by surfaces that satisfy the topological definition of an atom[2], the density at r may be equated to a sum of atomic source contributions S(r,)
Such a decomposition enables one to view the properties of the density from a new perspective and establishes the source function (SF) as a  potentially interesting tool to provide chemical information.
This study [3] is aimed at widening up the limited spectrum of applications of the SF [1,4] so as to increase the knowledge of  properties and usefulness of the SF.
We examine a number of cases, including (a) the effect of the X substitution on the source contribution from A to the electron density at bond critical point (BCP) in AX diatomics and the extent to which the near transferability of the integral properties of A, when present, is reflected in the source contribution from A to the BCP density; (b) the relative weight of the internal and external contribution to the density at a (3,-3) critical point when this is a non-nuclear maximum, rather than a maximum associated to a nuclear-cusp; (c) the relative weight of source contributions from the atoms of H-bond complex to the density at the H-bond CP, in a series of  complexes of increasing strength. The correspondence between the H-bond classification provided by the ELF topological approach and by the SF is also highlighted.   
The figure on the right, where circles are proportional to the S(r,) from different atoms  and with negative sources in light grey, clearly shows how the SF discriminates among very strong, intermediate and weak H-bond complexes.
It is concluded that the SF appears as a practical tool to unravel the local and non local character of the electron density distributions and to quantify such a locality and nonlocality in terms of a physically sound and appealing chemical partitioning.  	

References
1	Bader, R. F. W. and Gatti, C. (1998), Chem. Phys. Lett. 287, 233-238.
2	Bader,  R.F.W.  in Atoms in Molecules: A Quantum Theory, International Series of Monographs  on Chemistry, Oxford, University Press, Oxford (1990), 22.
3	Gatti, C., Cargnoni, F. and Bertini, L. (2003), J. Comput. Chem. 24, 422-436.
4	Overgaard, J.,Schiøtt, B., Larsen, F. K., Iversen, B. B. (2001) Chem. Eur. J.  7, 56-3767.