I'm studying about azobenzene. The S1(nπ*) and S2(ππ*) are important in transition from trans to cis and vice versa.
And, I'm looking for a way to get S1 and S2 excited states from simple Gaussian calculation (time-independent) DFT. To get the S1 and S2, I know we have to re-calculate TD-DFT.
Can I use the gap between HOMO and LUMO as S1 and the gap between HOMO-1 and LUMO as S2 approximately? I need any solutions without extra TD-DFT calculation.
To get a more reliable result, you usually have to optimize the excited states. Since you want it simpler (and consequently more unreliable), you can obtain the vertical excitation energy of S1 and S2 from the log file of your TD-DFT calculation. This is the transition with the highest % weight.
Rengel Cane Sia Thank you for answer of my question. I mean how to represent S1 and S2 without TD-DFT calculation. Just using simple DFT calculation of the ground states is not reliable as you said. But, how reliable is the information of the ground states with respect to TD-DFT results? My option of input file is 'b3lyp/6-31++g(d,p) opt=(...) polar' This give us the information under ground states.
I added one figure from the Book:
Spectroscopic Methods in Organic Chemistry, 2nd Edition 2007
By Manfred Hesse, Herbert Meier, Bernd Zeeh, M.
As I know, we should consider the energy difference coming from e-h interactions, etc. Then, how can we get the reliable approximation from HOMO and LUMO?
Do not expect any numerical accuracy from the analysis of ground state orbitals! You can, for illustrative purposes, consider excited states as a transitions from one orbital to another, but you need to know beforehand which orbitals to use. So you need at least one TD-DFT calculation or some additional insight.
I cannot imagine any situation when using ground-state orbitals give you some valuable result. Please tell us more about your problem. What do you mean when you say "represent" or "get" S1 state? Do you need excitation energy? Approximate electron density change? Orbital nature of the transition?
You can relate HOMO-LUMO gap with excitation energy. You can see the correlation if you are lucky and the energies are very different. Don't try to make any science from that.
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