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 cont= amination from higher states. There is no easy or cheap way out of this. You could use restricted open shell calculations, R= O 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 yo= u’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 ava= ilable 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 appr= oximation. Not published, just experience from a long project we never managed to finalize.

Best regards,

Per-Ola

From: owner-chemistry+per-ola.norrby=3D= =3Dastrazeneca.com\a/ccl.net [mailto:owner-chemistry+per-ola.norrby=3D=3D= 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->si= nglet excitations. In Gaussian, I can see that for each excitation, <S**= 2>=3D0.000.

While I calculate molecules like O2 which the ground= state is triplet, I need calculate triplet->triplet excitations by assi= gning TD=3Dtriplet .
In output, I found all kinds of <S**2> values (from 2 to 4). I am con= fused: 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 ab= sorption spectrum for the molecule?

Thank you very much!

Michael

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