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Dear
Michael, You get variable <S**2>
because you use the unrestricted approximation. For a triplet, the exact value
of <S**2> is 2; any value higher than that indicates contamination from
higher states. There is no easy or
cheap way out of this. You could use restricted open shell calculations, RO
instead of the default U (not sure about Gaussian, I’ve done those in
Jaguar), but in my experience, energies are less reliable even though
you’re guaranteed pure spin states. For
ground state calculations, you can project out most of the higher spins in the
unrestricted solution, but again, I’m not sure the method is available for
TDDFT. For accurate identification of “real” excited states,
I’ve used CASSCF/CASPT2, but even there,
the energies from unrestricted TDDFT can be as good or better. I did find the
CASPT2 calculations good for evaluating which of the DFT excited states were
“real”, and which were artifacts in the unrestricted approximation.
Not published, just experience from
a long project we never managed to finalize. Best
regards, Per-Ola From:
owner-chemistry+per-ola.norrby==astrazeneca.com\a/ccl.net
[mailto:owner-chemistry+per-ola.norrby==astrazeneca.com\a/ccl.net]
On Behalf Of Michael Morgan michaelmorgan937%gmail.com Dear All, Standard TDDFT calculations calculate singlet->singlet
excitations. In Gaussian, I can see that for each excitation,
<S**2>=0.000. While I calculate molecules like O2 which the ground state
is triplet, I need calculate triplet->triplet excitations by assigning
TD=triplet .
Thank you very much! Michael Confidentiality Notice: This message is private and may contain confidential and proprietary information. If you have received this message in error, please notify us and remove it from your system and note that you must not copy, distribute or take any action in reliance on it. Any unauthorized use or disclosure of the contents of this message is not permitted and may be unlawful. |