CCL:G: Broken Symmetry Singlet Excited State

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,


--- On Wed, 2/4/09, Cristian V. Diaconu cvdiaconu!A! <owner-chemistry]|[> wrote:

From: Cristian V. Diaconu cvdiaconu!A! <owner-chemistry]|[>
Subject: CCL:G: Broken Symmetry Singlet Excited State
To: "L, Sue " <chsue2004]|[>
Date: Wednesday, February 4, 2009, 2:15 PM

Sent to CCL by: "Cristian V. Diaconu"
 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,
 Sue L chsue2004(~) wrote:
 > Sent to CCL by: "Sue  L" []
 > 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
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