Subject: Summary: CHelpG calculation for Fe

From: Valentin Gogonea <[email-masked]>
Date: Thu, 28 Nov 2002 08:42:36 -0500

Dear CCLers,

A couple of weeks ago I asked for advice for selecting the radius for  
Fe(2+) for CHelpG calculations. Thank you very much to all who answered.

Valentin Gogonea
Cleveland State University

Here are the replies that I received:
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We never tried to go that far down in the periodic table when we  
developed some
of the radii for other atoms, but I suggest that you use a (consistent)  
value
that represents the van der Waals radius of the atom.  The danger of  
using too
large a radius is the loss of information from the other atoms in the  
molecule,
and an underdetermined charge fitting model.  Good luck with your
experimentation.  I suggest you try several reasonable values and then  
see if
the charges computed for all of the atoms are reasonably consistent.

Curt Breneman
RPI Chemistry

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 From gromacs list:

I did gaussian 98 calculations on Fe(III). Here are my notes
concerning the radii that I used. Hope it helps,

Cheers,
Marc

For iron there are several ionic radii. Let's use radii from
http://www.webelements.com/webelements/elements/text/Fe/radii.html.

o    Fe(III) 6-coordinate, octahedral 69 pm

o    Fe(III) 6-coordinate, octahedral, high spin 78.5 pm

Note: The "efective ionic radii" quoted here assume that the ionic
radius of F- is 133 pm and that of O2- is 140 pm. The values for iron
thus correspond quite well to che tabulated Gaussian Merz-Kollman
radii, and will be scaled for Francl and Breneman, with a mean factor
for F- and O2-. Values depend upon the coordination number and for
d-block metals on the fact whether or not the metal is in a high or
low spin state.

Atomnb        AtTyp         Merz-Kollman  Francl        Breneman
                                           (CHELP)       (CHELPG)
............. ............. ............. ............. .............
26            Fe            0.69          0.75          0.85
               6-coordinate,
               octahedral
26            Fe            0.785         0.85          0.97
               6-coordinate,
               octahedral,
               high spin

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you could try a number of different radii and see where the charges
are approximately constant. I would guess a radius of 2.0 A.
Please, see our article, where we determine a radius for Cu2+:
E. Sigfridsson & U. Ryde (1998) "A comparison of methods for deriving  
atomic charges from the electrostatic potential and moments".  J. Comp.  
Chem. 19, 377-395.

Best regards,
Ulf
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We typically use Fe radius of 1.5A, sometimes even 1.3A, but the Fe(3+)
or Fe(2+) is nearly always surrounded by ligands and the whole complex
is treated by quantum calc's (DFT), so the Fe radius is not important.
Use of the bare ion alone will lead to poor properties, as we showed
in an Inorg. Chem. paper on metal cations in water in 1996.
Best Regards,
Lou Noodleman
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    To read in a radius for Fe, use the option  
"pop=(chelpg,ReadAtRadii)". These values are usually not the textbook  
van der Waals radii, but radii that have been chosen by the authors of  
a given charge scheme. The choice of a radius for an element that was  
not in the original parameter set is a judgment call and depends  
somewhat on the task at hand (whether the atom is ionic, for example).  
Below is an example input file in which I arbitrarily chose to use the  
metallic radius of Fe.

#p rhf/sto-3g scf=tight symm=loose pop=(chelpg,readradii) test

ferrocene example

0 1
  Fe                -0.000000   -0.000000    0.000000
  X                 -0.000000    0.000000   -2.160000
  C                 -0.000000    1.250000   -2.160000
  C                  1.188821    0.386271   -2.160000
  C                 -1.188821    0.386271   -2.160000
  H                  0.113936    2.334029   -2.160000
  C                  0.734732   -1.011271   -2.160000
  H                  2.255002    0.612895   -2.160000
  C                 -0.734732   -1.011271   -2.160000
  H                 -2.184585    0.829614   -2.160000
  H                  1.279732   -1.955239   -2.160000
  H                 -1.464084   -1.821299   -2.160000
  X                  0.000000    0.000000    2.160000
  C                  0.000000    1.250000    2.160000
  C                 -1.188821    0.386271    2.160000
  C                  1.188821    0.386271    2.160000
  H                 -0.113936    2.334029    2.160000
  C                 -0.734732   -1.011271    2.160000
  H                 -2.255002    0.612895    2.160000
  C                  0.734732   -1.011271    2.160000
  H                  2.184585    0.829614    2.160000
  H                 -1.279732   -1.955239    2.160000
  H                  1.464084   -1.821299    2.160000

Fe 1.24

Regards,
Jim Hess