Hello Chris,
Thank you very much for your
reply. I am studying a reaction, in which the transition state contain
biradical character. I have tired to extract the energy of the pure
singlet from the broken symmetry state by using the spin-correction procedure
which was proposed by Yamaguchi et al (Chem. Phys. Lett. 1994,
231, 25–33)
E(singlet) = E(broken symmetry from
UDFT) + fSC[E(broken symmetry from UDFT – E(triplet from
UDFT))
Where fSC = 1<S^2> /
[3<S^2> - 1<S^2>]
S^2: expectation value of the total
spin
The S^2 before and after the
annihilation are listed in the Gaussian output. In my case,
Annihilation of the first spin
contaminant:
S^2 before annihilation 0.8631, after 0.4136
I am not sure that whether I should
use the S^2 before or after the annihilation for the projection method.
Best regards,
Sue
--- On Wed,
2/4/09, Cristian V. Diaconu cvdiaconu!A!rice.edu
<owner-chemistry]|[ccl.net> wrote:
From: Cristian V. Diaconu cvdiaconu!A!rice.edu
<owner-chemistry]|[ccl.net>
Subject: CCL:G: Broken Symmetry Singlet
Excited State
To: "L, Sue " <chsue2004]|[yahoo.com>
Date: Wednesday,
February 4, 2009, 2:15 PM
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-x-rice.edu
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