Subject: Atomic charges: Ni, Co

From: "Petr Toman" <[email-masked]>
Date: Fri, 11 Aug 2000 09:49:57 +0200

Dear CCLers,
I wonder if there is a suitable method for calculation of atomic charges 
in molecules containing Ni or Co. 
I tried Pop=MK in Gaussian 98, but...
 Merz-Kollman atomic radii used.
 GetVDW:  no radius for atom   1 atomic number  28.

Of course, it is possible to use Mulliken population analysis, but I am 
not sure if it is the best way.

Thanks a lot.

Best regards,
Petr Toman

---

From: Marcel Swart <[email-masked]>
Date: Fri, 11 Aug 2000 11:56:49 +0200

>not sure if it is the best way.

Sure there is.
We have recently developed a Multipole Derived Charge analysis,
which gives the most acurrate charge description possible in DFT.
(The paper has been accepted to appear in J.Comput.Chem.)

Since there are NO parameters like for instance the MK-radii,
as long as the calculation on the molecule is possible,
the charges can be obtained, and are the best description of
the charge distribution within that (DFT)-calculation.

It has been implemented in the ADF program.

Marcel Swart.

---

From: Jacco van de Streek <[email-masked]>
Date: Fri, 11 Aug 2000 12:41:33 +0200

>  GetVDW:  no radius for atom   1 atomic number  28.

This seems to be related to a recent question about the same topic.

>From my own experience, best results for partial charges are obtained
by:

a. Using the set sampling points of Breneman & Wiberg (the 'ChelpG'
keyword).
b. Restricting the dipole moment (the 'Dipole' keyword).

The command line then reads something like:
# HF/6-31G** Pop=(ChelpG,Dipole) etc.

The reference to the Breneman & Wiberg paper is:
Breneman, C.M. & K.B. Wiberg (1990). J. Comp. Chem. 11, 361-373.

ChelpG is the only method which uses a lot of sampling points, and it
can cope with additional charge centres (MK can't in my experience,
Gaussian crashes).

Restricting the dipole moment has two advantages:

a. At long distances, the monopole of a neutral molecule is neglegible
(=zero), and the dipole interactions are next in importance.
b. There are quite a lot of degrees of freedom, which can render some
pairs of atoms ill-defined (dependent on each other). Restricting the
dipole moment might not solve this problem completely, but at least will
push the calculation in the right direction.

If anyone can think of a disadvantage, I'd like to know.

If the molecule contains atoms for which Gaussian has no atomic radius
available, you must split your job (otherwise Gaussian crashes):

1. Run the optimisation / single point job, specifying a checkpoint
file.
2. Run the ESP job.

The command line for the second step reads:
# HF/6-31G** Pop=(ChelpG,Dipole,ReadRadii) Density=CHK Geom=checkpoint
etc.

This indicates that both the electron density and the molecular geometry
must be read from the checkpoint file.

The 'ReadRadii' keyword indicates that at the end of the input radii for
the ChelpG calculation are provided. The format of the radii is:
Blank line
'Atomic number' 'radius in Angstrom'
Blank line
Blank line

So for the calculation of the a molecule containing bromine, after
having run the single point calculation, the entire input file is as
simple as:

*****************************************
$ RunGauss
%Chk=C14H28Br2.chk
# HF/6-31G** Pop=(ChelpG,Dipole,ReadRadii) Density=CHK SCF=Direct
Geom=(checkpoint,NoDistance,NoAngle)

1,14-dibromotetradecane

0 1

 35 1.85

*****************************************

I hope it is clear that your optimisation / single point job must
contain '%Chk=C14H28Br2.chk' as its second line.

Comments and experiences welcomed.

Hope this helps,
-- 
Jacco van de Streek (mailto:[email-masked])
Dept. of Solid State Chemistry
University of Nijmegen
The Netherlands