This is some curiosity for TDDFT calculation in gaussian 09.
Recently, I have tried to calculate doublet to doublet transition property using TDDFT option in gaussian 09. There is only option for singlet and triplet in TDDFT. However, I found that some excitation shows the S2 value of 0.75 ~ 0.9/.
I think that near 0.75 values may represent the doublet to doublet transition even if gaussian 09 cannot support this kind of calculation using TDDFT.
I already know that some methods (CASSSCF, SAC-CI, etc) can be used to calculate doublet-doublet transition. But my system is too large to apply those methods.
To the best of my knowledge it is possible to do TDDFT calculations for systems with doublet multiplicity with gaussian. You just have to specify the multiplicity of your system as usual above the coordinates in the input file. The TDDFT options "singlet" and "triplet" only apply to closed shell systems.
You just have to check, that the S^2 expectation value is not too far away from 0.75, because otherwise your state will be spin-contaminated and the results will thus not be very meaningful.
Thanks Christof
I've understood that singlet is default option thus i'm a little bit confused.
Now i can exactly understand that current TDDFT cannot well describe doublet to
doublet excitation property.
Thank for you helping.
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