CCL: regarding negative fukui function


Most likely your problem is the use of the Mullikin charges.  They give unrealistic and incorrect values for charges.Take a look at

Condensation of Frontier Molecular Orbital Fukui Functions by Bulat et al. J. Phys. Chem. A 2004, 108, 342-349

"Possible negative values for the Fukui indexes have been
discussed in the literature and considered as artifacts coming
from the condensation procedure12 or due to strong structural
distortions14 (see also refs 13 and 15-17).
Thanikaivelan et al.18 have thoroughly studied the effects of
the population scheme and the basis set dependency of the Fukui
indexes within the finite difference approximation eqs 9-11.
The most stable schemes upon basis set modification turn out
to be the Atoms in Molecules (AIM)19 and the CHELPG
(charges from electrostatic potentials, using a grid-based
20 this last one being based on the molecular electrostatic
potential (MESP) to derive the atomic charges. Interestingly
enough, these two schemes provide also very similar Fukui
indexes and they both predict appropriate reactivity sites for
all kinds of attacks (nucleophilic, electrophilic, and radical).
Although they note that Hirshfeld partitioning provides in all
studied cases nonnegative Fukui indexes, as also stated by Roy
et al.,12 this fact cannot be generally extended since the
allowance of negative values is in part a consequence of the
finite difference approximation. Indeed, eqs 3-5 might be
negative at some points since FN+1(r) might be smaller than
FN(r) and FN(r) might be smaller than FN-1(r). Hence, regardless
of the condensation scheme chosen negative Fukui indexes
might appear. Furthermore, Hirshfeld partitioning fails to predict
the correct reactivity trends in some cases.12,18"

and their references

On Tue, Sep 11, 2012 at 11:39 AM, AMBRISH KUMAR SRIVASTAVA ambrishphysics]^[ <owner-chemistry[A]> wrote:

Sent to CCL by: "AMBRISH KUMAR SRIVASTAVA" [ambrishphysics_._" target="_blank">]
Respected all,
During calculation on fukui fn using mullikan atomic charges, i got a number of negative values for e.g. f0 values are given below:
how to interpret these values?? how can i extract any useful info with this data set??
any kind of help will be highly appreciated!
thanking you in advance..
Ambrish K. Srivastava

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Ronald L Cook
Principal Scientist
TDA Research, Inc.