CCL: symmetry breaking



Dear Susi, Oleg, ans Mikael,
Thank you for your responses, which were very helpful.
I have succeeded to find the broken symmetry solutions with wavefunction stability checks.
In short, there was no correlation between ease of finding the broken symmetry solution and biradical character of the final broken symmetry solution. There seems to be a correlation with the amount of change in electron density, at least for the few systems I studied.
I have a follow-up question: Ess et al. (J. Chem. Phys. A, 2011,115,77) used fractional spin occupation DFT (FS-DFT) to analyze broken symmetry solutions without spin contamination. They start from the closed-shell singlet orbitals and change the occupancies of HOMO and LUMO to 0.5 each, then do a SCF calculation with fixed occupations.
I am trying to figure out whether this can give the exact solution as claimed. As the closed-shell orbitals are symmetric, this will not give alpha and beta electrons different space orbitals. In UHF the HOMO and LUMO form linear combinations which allow alpha and beta electron to localize at opposite ends of the molecule.  I always considered this as an essential driving force for symmetry breaking. Can anyone comment on this?
Thanks,
Ulrike

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Assoc. Prof. Ulrike Salzner
Department of Chemistry
Bilkent University
06800 Bilkent, Ankara