

Dear Miquel,

Although I do not know exactly the reason for such behaviour of the
electron density Laplacian in CO molecule, I have some comments to
this subject:

1. Positive values of the Laplacian are typical of C-O multiple bonds
   as well as of C-F and some other bonds which are normally treated
   as covalent. The most famous example is F2 where the bond is defi-
   nitely purely covalent. Probably the positive Laplacian can occur
   in "electron-rich" molecules.
2. The Bader's topological theory does not state that the bonds with
   the positive Laplacian are ionic ones. Bader distinguishes between
   covalent and "closed-shell" interactions. The latter include ionic,
   donor-acceptor, and van der Waals interactions.
3. It is quite incorrect to compare the absolute values of the Lapla-
   cian in the bonds formed by different atoms (one cannot say whether
   LiF is more or less ionic than CO)
4. There exists another criterion of covalency suggested by Cremer and
   Kraka. Namely, a bond is to be considered as covalent, if the elec-
   tronic energy density is negative. In CO molecule this is the case.
   I would not like to discuss whether or not the Cremer-Kraka's cri-
   terion is better than Bader's one.
5. I have repeated you calculation (QCISD/6-311++G**) of CO molecule
   at C-O separation of 1 angstroem using Gaussian92 followed by Ba-
   der's EXTREME program and obtained the value of the electron den-
   sity Laplacian at the (3,-1) critical point as much as +2.837au.
   Thus, either in my or in you calculation something is wrong.

                              Yours sincerely,
                         Sergei Vyboishchikov
                         FB Chemie der Universitaet Marburg
                         Hans-Meerwein-Strasse
                         D-35032 Marburg, Germany
                         E-Mail: [email-masked]