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### Topic: Help with TD-DFT calculations - Gaussian  (Read 2956 times)

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#### mendeleiev

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##### Help with TD-DFT calculations - Gaussian
« on: December 17, 2023, 04:59:29 AM »
Hello all,

I hope someone here can help me with some TD-DFT calculations. I am trying to calculate the excited state structures and energies of some Germanium compounds. Firstly, I run a TD calculation of the first 10 excited states, singlets and triplets, using:

TDA=(Nstates=10,50-50) WB97XD/genecp

From this output I see the first excites states energies and they look ok to me, such as:

Excited State   1:      Triplet-A      2.4156 eV  513.26 nm  f=0.0000  <S**2>=2.000
113 ->120        -0.18260
119 ->120         0.65189
This state for optimization and/or second-order correction.
Total Energy, E(CIS/TDA) =  -1629.19629051
Copying the excited state density for this state as the 1-particle RhoCI density.

Excited State   2:      Singlet-A      3.3296 eV  372.37 nm  f=0.0537  <S**2>=0.000
113 ->120        -0.18586
119 ->120         0.66543

I then try to optimise the first singlet and triplet as I want to see how the spin density looks like in those species. For the first triplet I just indicate multiplicity 3 in the gaussian input (0 3 right before the coordinates), and I get something that I think it makes sense, the molecule geometry changes and I get alpha and beta orbitals, one electron seems to be occupying a virtual orbital by himself.... etc

The problem comes with the first singlet excited state, in order to calculate it I use:

TD=(Nstates=6,50-50,root=2) WB97XD/genecp

Based in the previously calculates excited states, in which the second state is the first singlet. However, after the calculation was done (it took quite a long time) I see an output with completely normal orbitals (not alpha and beta), a nice homo with two electrons and a empty LUMO, as in the molecule looks like a normal diamagnetic ground state molecule.

Does someone see something weird in my procedures? If you need any more details please let me know. My apologies if I am making some very basic mistakes I am very new in TD-DFT. Thank you all in advance!

#### Corribus

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##### Re: Help with TD-DFT calculations - Gaussian
« Reply #1 on: December 18, 2023, 09:42:49 AM »
We don't have any experts here who are knowledgeable in computational chemistry, so you will probably receive no reply to this. My advice is to post this at a specialized forum for the software you are using.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman