Subject: Re: calculating atomic charges with AIMPAC

From: Wai-To Chan <chan*at*curl.gkcl.yorku.ca>
Date: Thu, 29 May 2003 02:09:19 -0400 (EDT)

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I currently have an undergraduate student who is attempting to compare 
charges of methylated benzene and ferrocene systems using the AIMPAC 
software.  In the case of hexamethyl benzene and the ferrocene analogues he 
is getting very large negative charges on some the ring carbons (-5 to -20) 
and small positive charges on iron (0.5) Can anyone tell me if there is a 
problem with the program or if the ring system can not be treated with this 
method?

   I remember reading about the tendency of AIM changes to be larger in magnitude
than expected on the basis of chemical intuition. There is a paper by
Kenneth Wiberg from the early 90s in J. comp. chem. about the comparison of 
several atomic-charge partitioning scheme including Bader's you may find useful. 
You may also consider using the Natural atomic charges method if you want 
faster and less 'surprising' results. 

Wai-To Chan

---

From: "Shobe, Dave" <dshobe{at}sud-chemieinc.com>
Date: Thu, 29 May 2003 08:54:29 -0400

But a charge of -20 almost certainly is the result of assigning the
electrons of one atom's basin to another atom.  Something which occurs as a
result of the program failing to find the boundary between them.  

At any rate, I would try a 2nd AIM calculation program if one is available,
before giving up.  E.g., I often find Gaussian's AIM code halts with a
(usually cryptic) error message, but for the same molecule AIM2000 may
complete the calculation with no problem.

-----Original Message-----
From: Wai-To Chan [mailto:chan{at}curl.gkcl.yorku.ca]
Sent: Thursday, May 29, 2003 2:09 AM
To: chemistry{at}ccl.net
Subject: CCL:calculating atomic charges with AIMPAC

   I remember reading about the tendency of AIM changes to be larger in
magnitude
than expected on the basis of chemical intuition. There is a paper by
Kenneth Wiberg from the early 90s in J. comp. chem. about the comparison of 
several atomic-charge partitioning scheme including Bader's you may find
useful. 
You may also consider using the Natural atomic charges method if you want 
faster and less 'surprising' results. 

Wai-To Chan

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