Subject: Re: CCL: CO Charge distribution

From: Marcel Swart <m.swart<<at>>few.vu.nl>
Date: Wed, 21 Apr 2004 20:47:26 +0200

I would also like to contribute a little to this discussion, as CO is a
molecule I have studied in the past with my Multipole Derived Charge 
analysis
derived from an atomic multipole expansion (J.Comput.Chem. 22 (2001) 
79-88).
The MDC analysis represents by construction the atomic multipoles that, 
in turn,
result directly from the charge density; e.g. an accurate description 
of the
electrostatic potential is obtained (for more details see the paper).

The point now is that the atomic multipoles are reconstructed up to a 
certain
level, e.g. the MDC-d charges represent the multipoles up to the dipole 
and
the MDC-q charges up to quadrupole (note: the atomic AND molecular 
multipoles
are reproduced). Sometimes (due to symmetry) it is impossible to 
reproduce
the multipoles due to symmetry (best example is H2, where any charge 
analysis
will give a charge of zero), which can be resolved by adding a third 
(dummy)
point, for instance in the bond-midway point.

So far the theory, now the practicalities: using Becke-Perdew in a TZ2P 
basis
with the ADF program, you get of course a reasonable dipole moment 
(0.18D),
which is represented by the MDC-d charges (+0.04 on O, -0.04 on C); on 
the
other hand, by adding the third (XX) charge in the bond-midway point, 
one
reconstructs both the dipole and quadrupole moment and finds charges of
-0.85 (O), -0.54 (C) and +1.39 (XX). (note: represented dipole -0.07 
a.u.;
represented quadrupole (zz) -1.52 a.u.; exp. values -0.04 and -1.86 
a.u.;
the MDC-d charges give a value for quad.zz of only -0.03 !)

In summary, the MDC analysis gives the correct positive O and negative 
C (MDC-d),
on the other hand is consistent with sophisticated techniques like AIM 
that
indicate a larger negative basin around O (as apparent from the MDC-q 
charges).

On Wednesday, Apr 21, 2004, at 16:26 Europe/Amsterdam, Errol Lewars 
wrote:

> Reproducing the dipole moment of carbon monoxide is a known problem in
> computational chemistry. See e.g. C. J. Cramer, Essentials of
> Computational Chemistry, pp. 268-269. The experimental moment is 0.11 D
> (with C negative, apparently, from molecular beam resonance
> measurements). An accurate calculation used the DFT B3LYP functional 
> and
> a big basis set (A. J. Cohen and Y. Tantirungrotechai, Chem Phys Lett,
> 1999, 299, 465).

On Wednesday, Apr 21, 2004, at 16:41 Europe/Amsterdam, Jamie Platts 
wrote:

>   It's not just Mulliken charges! Any charge partitioning scheme will
> give a positive C and negative O (except obviously for those 
> constrained
> to reproduce the dipole such as Merz-Kollman or CHELPG). Thus Atoms-in-
> Molecules, Natural Population Analysis, Lowdin decomp, etc all agree on
> the sign, if not the size, of the atomic charges.
>
>   Bader gives a nice explanation of this apparent anomaly in his book
> "Atoms in Molecules: A Quantum Theory". Essentially, the C and O 
> electron
> densities are polarised within their own basins by the charge transfer
> from C to O, leading to two contributions (inter- and intra-atomic) to 
> the
> total dipole moment. CO is a notable case as these are of almost equal
> magnitude but opposite sign.
>

––––––––––––––––––––––––––––––––––––––––––––
dr. Marcel Swart

Organische en Anorganische Chemie (until May 1, 2004)
Theoretische Chemie (after May 1, 2004)

Vrije Universiteit Amsterdam
Faculteit der Exacte Wetenschappen

De Boelelaan 1083
1081 HV Amsterdam
The Netherlands

E   m.swart<<at>>few.vu.nl
W  http://theochem.chem.rug.nl/~swart
––––––––––––––––––––––––––––––––––––––––––––