From owner-chemistry@ccl.net Fri Nov 26 09:28:00 2010
From: "Herbert Fruchtl herbert.fruchtl[#]st-andrews.ac.uk" <owner-chemistry##server.ccl.net>
To: CCL
Subject: CCL:G: Question on calculation of charge center of electrons in a molecule
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X-Original-From: Herbert Fruchtl <herbert.fruchtl[a]st-andrews.ac.uk>
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Date: Fri, 26 Nov 2010 14:25:01 +0000
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Sent to CCL by: Herbert Fruchtl [herbert.fruchtl::st-andrews.ac.uk]
The centre of charge of each Gaussian function is its origin, i.e. the nucleus 
if you have atom-centred basis functions. The Mulliken charge is the sum of 
those functions, multiplied with their occupation number. Shouldn't this sum 
have its centre on the nucleus as well? And the sum of all of them (over all 
atoms) gives the complete charge distribution. Where am I going wrong?

   Herbert

Ulf Ekström ulfek(~)few.vu.nl wrote:
> Sent to CCL by: =?ISO-8859-1?Q?Ulf_Ekstr=F6m?= [ulfek],[few.vu.nl]
> On Thu, Nov 25, 2010 at 9:56 AM, Herbert Fruchtl
> herbert.fruchtl-.-st-andrews.ac.uk <owner-chemistry . ccl.net> wrote:
>> Sent to CCL by: Herbert Fruchtl [herbert.fruchtl*_*st-andrews.ac.uk]
>> I don't have time to look at the maths of it, but I think the centre of
>> charge of the electron density should be identical to the centre of the
>> Mulliken charges, which would be easy to calculate (analogous to the centre
>> of mass).
> 
> This will not work, for example if you have a single centre basis set all
> Mulliken charge would be on that center. What you should do is to compute
> the dipole moment of the molecule and use that to work out the center
> of charge.
> 
> Sincerely,
> Ulf Ekstrom> 
> 

-- 
Herbert Fruchtl
Senior Scientific Computing Officer
School of Chemistry, School of Mathematics and Statistics
University of St Andrews
--
The University of St Andrews is a charity registered in Scotland:
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