Triplet excited state with SA-SF-DFT

Questions
#1

Dear all,

I would like to compute the singlet and triplet excited state energy for a closed-shell system using the Spin-Adapted Spin-Flip DFT approach. While the calculation of the singlet states goes smoothly, I am not able to get the triplets. By including the keyword “CIS_TRIPLETS = true” in the $rem section I always obtain the energy for singlet states, even if I write “CIS_SINGLETS = true”. Is there a way to obtain the triplets within this approach?

Thank you very much for your help.

Best Regards,

Gaetano

#2

Could you please post your input file that shows the problem?

#3

That code doesn’t respond to CIS_TRIPLETS. To get a triplet (S=1), you need to start from a quintet reference configuration (S=2).

#4

Thank you very much for your help!

#5

Hi.
Sorry to hack this conversation, but I have a similar query.
If I need to compute the singlet-triplet energy difference of a molecule, how to calculate it using the SA-SF DFT method?

#6

Ground state singlet and lowest triplet? Or is one or both states an excited state?

#7

Yes in one molecule, the ground state is the singlet and the triplet is the excited state and in the other, the ground state is the triplet and the excited state is a singlet.
These are basically diradicals.

#8

The way that the SA-SF-TDDFT method works is that it computes excited states by spin blocks, starting from a high-spin reference state (total spin quantum numbers S, say) to get states with total spin S=1 (e.g., triplet reference for singlet states). So the precise answer to your question is that you can’t get both energy differences from a single calculation. Is your ground state open-shell? From your description it seems like what you want in both cases are the lowest-energy S=0 and lowest-energy S=1 states, setting aside which of these is lower in energy. If so, my inclination would be to compute both of these as ground-state calculations with different spin multiplicities, at same level of DFT theory, and take the ground-state energy difference as your singlet-triplet gap.

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#9

Thanks for your reply.
In this case, will we get the S-T gap that is free from spin-contamination which is prevalent in the SF-DFT method?

#10

Maybe I’m still misunderstanding your question but my suggestion was to run both states as ground-state calculations with different multiplicities, and not as spin-flip. If the S=0 state is open-shell then it may be heavily spin contaminated (in which case you could consider using Yamaguchi’s approximate spin projection (OPSING=TRUE), but if it’s closed shell then you may see minimal spin contamination.

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