This is in response to the following comment which was made by a reviewer for a manuscript on which we are working.
"
The discussion on the absorption spectra should include the nature of the transitions. To do so, the CI description (nature and %) from the TD-DFT calculations is needed, and should be included on Table 1. Then, the bands can be described as CT states, or localized excitations (LE) according to the nature of the MOs involved. For each pair of compounds, the authors should discuss how fluorination changes the relation between the LE and the CT states, in terms of energy and intensity. "
Dear colleague
If you take a look at the output file you'll find in the TD-DFT all the states (labeled as "Excited State X") and their corresponding excitations just below. Take a look to the attached image, it's an example of what you should look for.
In the image you can see that the first excited state is 10.1347 eV above the ground-state and corresponds purely to a excitation from orbital 7 to orbital 9. Then you have to look in the .chk file to see which orbitals are those to identify the type of excitation (CT, localized...)
And the coefficients of each excitation are next to them, right-hand side. In the image case it was a closed-shell calculation, so the coefficients for the alpha and beta spins are the same, but only one is written. In this case, 0.70674^2(alpha)+0.70674^2(beta)=1, so everything is correct.
To sum up, you need to have the .chk file to identify the excitations. If you don't have it you'll need to repeat the calculation including the chk route and the keyword guess=save.
Hope you find it helpful
I completely understand how to use the coefficents as well as the formchk file to identify the label for a given excitation (i.e. HOMO -> LUMO). I am still lost as to how you can label the type of excitation. I have downloaded the formchk file to my local machine from a cluster and do not see how you can look at the fact we are going from HOMO -> LUMO and justify whether it is a local excitation or charge transfer type of transition.
As Andy said in his first paragraph, to identify the type of excitation take a look to the MOs involved in each excitation. Do they belong to the same region of the system? It is a localized excitation. Do they belong to different region? It is charge transfer.
Imagine a transition metal complex. An excitation from the d orbitals of the metal to the pi* orbitals of a ligand would be charge transfer. However, a pi->pi* excitation in the ligands would be a localized excitation.
I think I understand now and unfortunately I only have the 2nd edition of the book rather than the 3rd so I couldn't refer to your suggested text.
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