CCL:G: Broken Symmetry Singlet Excited State

 Sent to CCL by: "Cristian V. Diaconu" [cvdiaconu~!]
 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,
 Cristian V. Diaconu
 Postdoctoral Research Associate
 Department of Chemistry - MS60
 Rice University
 PO Box 1892
 Houston, TX 77251-1892
 Phone: 713-348-3734