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
aper
"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 method),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"
Sent to CCL by: "AMBRISH KUMAR SRIVASTAVA" [ambrishphysics_._http://gmail.com" target="_blank">gmail.com]
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:
-0.82305
-0.90080
0.22285
-0.14005
0.32460
-0.12940
0.58775
-0.13240
-0.45095
-0.12075
0.95505
-0.12835
-0.13485
0.36405
-0.03600
-0.24670
-0.15305
-0.05770
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
ambrishphysics]~[gmail.com