

Dear Miguel,

I'm not absolutely sure about this, but I would suspect more negative
values for the laplacian at least for CO. Your are right: positive
values should indicate a more inionic interaction. I carried out 
calculations on organolithium compounds, and the laplacian at the
bond critical point for the C-Li bond was usually in the area of +0.2,
Li-F interactions in these molecules were about +0.3. May be there is
a bug in the program? 
G. Frenking from Marburg pointed me to the fact that the energy density
(H(rc)) at the bond critical point is a better measure for the covalency
of the respective bond. H(rc) is defined as the sum of kinetic energy
density (G(rc)) and potential energy density (V(rc)). For covalent bonds
it is always negative, whereas ionic, H-bond and vdW interactions have
positive densities! ref. D. Cremer, E. Kraka, Croat. Chem. Acta 1984,57,
1259.
Furtheron Cioslowski in Tallahassee calculates degrees of ionicity starting
from the Bader integrations (J. Cioslowski, S.T. Mixon, JACS 1991,113,4142.

Hope this helps

John

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Dr. John Lohrenz
Dept. of Chemistry                         Phone: (403) 220 3232
University of Calgary                      FAX:   (403) 289 9488
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