Dear Per-Ola,

Thank you very much for = your kind input!

Now I remember Gaussian = manual states the TDDFT calculations are only effective for close shell = systems. Probably I need look at other methods as you suggested = CASPT2.

Thank = you.

Best = regards,

Michael

From: = owner-chemistry+michaelmorgan937=3D=3Dgmail.com _ ccl.net = [mailto:owner-chemistry+michaelmorgan937=3D=3Dgmail.com _ ccl.net] On = Behalf Of Norrby, Per-Ola = Per-Ola.Norrby^^astrazeneca.com
Sent: Wednesday, August 17, = 2016 1:48 AM
To: Morgan, Michael = <michaelmorgan937 _ gmail.com>
Subject: 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=3D=3Dastrazeneca.com=CCl.net = [mailto:owner-chemistry+per-ola.norrby=3D=3Dastrazeneca.com=CCl.net] = On Behalf Of Michael Morgan = michaelmorgan937%gmail.com
Sent: den 17 augusti 2016 = 05:42
To: Norrby, Per-Ola = <Per-Ola.Norrby=3Dtrazeneca.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>=3D0.000.

While I = calculate molecules like O2 which the ground state is triplet, I need = calculate triplet->triplet excitations by assigning TD=3Dtriplet . =
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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