From owner-chemistry:~at~:ccl.net Wed Feb  4 07:59:00 2009
From: "Cristian V. Diaconu cvdiaconu!A!rice.edu" <owner-chemistry.:.server.ccl.net>
To: CCL
Subject: CCL:G: Broken Symmetry Singlet Excited State
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X-Original-From: "Cristian V. Diaconu" <cvdiaconu*o*rice.edu>
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Date: Wed, 04 Feb 2009 00:15:32 -0600
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Sent to CCL by: "Cristian V. Diaconu" [cvdiaconu~!~rice.edu]
Hello Sue,

The total energy reported by Gaussian is not for 'annihilated' wave function, but 
for the single determinant that it uses in the Kohn-Sham scheme (for DFT there 
isn't really any wave function).  There are ways to extract the energy of the pure 
singlet from the broken symmetry state, but for DFT they only work in certain 
cases.  Please look at discussions in:

J. Chem. Phys. 121, 10026 (2004)
J. Chem. Phys. 74, 5737 (1981)
Coord. Chem. Rev. 238–239, 187 (2003)
C. J. Cramer, in Essentials of Computational Chemistry. Theories and Models. 
(Wiley, Chichester, 2002), Chap. 14.4, pp. 456 – 459.

For example, for a molecule with two unpaired electrons on two sites which 
interact weakly (e.g., H2 molecule stretched, with a large bond length, say 10 
Angstroms), if <S^2> ~ 1,

E(singlet) = 2 E(broken sym) - E(triplet)

I hope this helps.  If you have further questions, please include a little detail 
on the system you are looking at.

Best regards,
Chris

Sue L chsue2004(~)yahoo.com wrote:
> Sent to CCL by: "Sue  L" [chsue2004..yahoo.com]
> Hi, 
> 
> I am running a DFT open-shell singlet calculation using Gaussian 03 to obtain a broken symmetry singlet excited state. Does anyone know whether the electronic energy calculated at the end based on the annihilated wave function or not? 
> 
> Thank you very much!
> 
> Best regards,
> Sue> 
> 
> 
> 

-- 
Cristian V. Diaconu
Postdoctoral Research Associate
Department of Chemistry - MS60
Rice University
PO Box 1892
Houston, TX 77251-1892
Phone: 713-348-3734
Email: cvdiaconu:-:rice.edu