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Transition energies using TD-DFT

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Member
Registered: Dec 2012
Posts: 3
When doing a TD-DFT calculation, computing the first 10 transitions using the "50-50" keyword to include singlets and triplets, I got convergence that included negative transition energies?!? I would think that this is a meaningless results, but what DOES it mean? Is this a result of some sort of orbital energies calculated wrong due to something?

Has anyone out there ever seen anything like this? Any help or insight would be greatly appreciated!

Thanks
Member
Registered: Aug 2008
Posts: 43
Are these negative excitation energies singlets or triplets? I have seen this sometimes with excited triplet states. You can try using "tda" instead of "td" in g09 if you have revision d01. This invokes the tamm-dancoff approximation, which helps mitigate such problems (see the below paper). You probably also want to make sure your ground state wavefunction is stable by using "stable(opt,nstates=12)", and you should try several #s for nstates until the larger #s give you the same resulting wavefunction.

Peach, M. J. G.; Williamson, M. J.; Tozer, D. J. J. Chem. Theory Comput. 2011, 7, 3578–3585.

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