From owner-chemistry@ccl.net Sun Nov 28 21:47:00 2010 From: "Jiabo Li jiaboli/./yahoo.com" To: CCL Subject: CCL:G: Question on calculation of charge center of electrons in a molecule Message-Id: <-43236-101127210227-22698-pXn/U8kCPHb06EGaeoAIZg,server.ccl.net> X-Original-From: Jiabo Li Content-Type: multipart/alternative; boundary="0-160776249-1290909739=:87851" Date: Sat, 27 Nov 2010 18:02:19 -0800 (PST) MIME-Version: 1.0 Sent to CCL by: Jiabo Li [jiaboli : yahoo.com] --0-160776249-1290909739=:87851 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable To understand what is wrong, you can look at a simple example: the hydrogen= atom in its excited state with the single electron on a SP hydride orbital= . In this case, the electron density is not central symmetric but polarized= , and the dipole moment is not zero (a big dipole moment), while in this ca= se the net atomic charge is zero (neutral). =A0 Jiabo --- On Fri, 11/26/10, Herbert Fruchtl herbert.fruchtl[#]st-andrews.ac.uk wrote: > From: Herbert Fruchtl herbert.fruchtl[#]st-andrews.ac.uk Subject: CCL:G: Question on calculation of charge center of electrons in a = molecule To: "Li, Jiabo " Date: Friday, November 26, 2010, 6:25 AM 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 nucl= eus if you have atom-centred basis functions. The Mulliken charge is the su= m of those functions, multiplied with their occupation number. Shouldn't th= is 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 wr= ong? =A0 Herbert Ulf Ekstr=F6m ulfek(~)few.vu.nl wrote: > Sent to CCL by: =3D?ISO-8859-1?Q?Ulf_Ekstr=3DF6m?=3D [ulfek],[few.vu.nl] > On Thu, Nov 25, 2010 at 9:56 AM, Herbert Fruchtl > herbert.fruchtl-.-st-andrews.ac.uk 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 cen= tre >> of mass). >=20 > 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. >=20 > Sincerely, > Ulf Ekstrom>=20 -- 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: No SC013532 -=3D This is automatically added to each message by the mailing script =3D-=A0 =A0=A0=A0http://www.ccl.net/cgi-bin/ccl/send_ccl_message=A0 =A0=A0=A0http://www.ccl.net/cgi-bin/ccl/send_ccl_message Subscribe/Unsubscribe:=A0 =A0 =A0 http://www.ccl.net/chemistry/sub_unsub.sh= tmlConferences: http://server.ccl.net/chemistry/a= nnouncements/conferences/=A0 =A0=A0=A0http://www.ccl.net/spammers.txt--0-160776249-1290909739=:87851 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable
To understand what is wrong, you can loo= k at a simple example: the hydrogen atom in its excited state with the sing= le electron on a SP hydride orbital. In this case, the electron density is = not central symmetric but polarized, and the dipole moment is not zero (a b= ig dipole moment), while in this case the net atomic charge is zero (neutra= l).
 
Jiabo

--- On Fri, 11/26/10, Herbert Fruchtl herbert.fruchtl[= #]st-andrews.ac.uk <owner-chemistry,ccl.net> wrote:

From: Herbert Fruchtl herbert.fruchtl[#]st-andrew= s.ac.uk <owner-chemistry,ccl.net>
Subject: CCL:G: Question on calc= ulation of charge center of electrons in a molecule
To: "Li, Jiabo -id#3= p9-" <jiaboli,yahoo.com>
Date: Friday, November 26, 2010, 6:25 AM<= BR>

Sent to CCL by: Herbert Fruchtl [herbert.fruchtl= ::st-andrews.ac.uk]
The centre of charge of each Gaussian function is it= s origin, i.e. the nucleus if you have atom-centred basis functions. The Mu= lliken charge is the sum of those functions, multiplied with their occupati= on number. Shouldn't this sum have its centre on the nucleus as well? And t= he sum of all of them (over all atoms) gives the complete charge distributi= on. Where am I going wrong?

  Herbert

Ulf Ekstr=F6m ulfe= k(~)few.vu.nl wrote:
> Sent to CCL by: =3D?ISO-8859-1?Q?Ulf_Ekstr=3DF= 6m?=3D [ulfek],[few.vu.nl]
> On Thu, Nov 25, 2010 at 9:56 AM, Herbert= Fruchtl
> herbert.fruchtl-.-st-andrews.ac.uk <owner-chemistry . c= cl.net> wrote:
>> Sent to CCL by: Herbert Fruchtl [herbert.fruc= htl*_*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, whi= ch would be easy to calculate (analogous to the centre
>> of mass)= .
>
> This will not work, for example if you have a single cen= tre 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,<= BR>> Ulf Ekstrom>

-- Herbert Fruchtl
Senior Scientific Com= puting Officer
School of Chemistry, School of Mathematics and Statistics=
University of St Andrews
--
The University of St Andrews is a cha= rity registered in Scotland:
No SC013532



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