CCL:G: triplet TDDFT calculation



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
Sent: den 17 augusti 2016 05:42
To: Norrby, Per-Ola <Per-Ola.Norrby\a/astrazeneca.com>
Subject: CCL:G: triplet TDDFT calculation

 

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 .
In output, I found all kinds of <S**2> values (from 2 to 4). I am confused: for those are not close to 3, are they still spin-allowed therefore
experimentally observable? Should I include them if I want to predict an absorption spectrum for the molecule?

 

Thank you very much!

 

Michael


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