You didn't indicate your material complex. Generally, the first excited state is just after the ground state above the energy gap. This may the bottom of princple conduction band Ec in solids or HOMO in molecular structures

Dear Moly Crist ,

Concerning TD-DFT, it depends at first on your system. What are you studying ?

After, the functionnal and basis-set are very important. Generally people perform a benchmark with a post-HF method (DLPNO-CCSD(T) in Orca for example) to be sure to use a proper functionnal. If you use Gaussian, I think the TDA approximation cannot be set to "false" and some inconsistencies concerning intensities of excitations could occur. Also, a very fine grid is advised, depending on the version of Gaussian, you might indicate to use "Int=ultrafine" which, I think, it is not the standard in Gaussian 09.

Depending on what you want to study (MO involved, type of excitation etc ...) Post-analysis is important. I could suggest you to have a look on the powerful software named Multiwfn (free).

I give you some articles that I hope, could help you:

Article TD-DFT Performance for the Visible Absorption Spectra of Org...

Article Extensive TD-DFT Benchmark: Singlet-Excited States of Organi...

Article ChemInform Abstract: The Calculations of Excited-State Prope...

Article Dye chemistry with time-dependent density functional theory

Article How accurate are TD-DFT excited-state geometries compared to...

In fact, two factors are important- transition probability and oscillator strength.

See the link

Preprint The experimental and simulated UV–Vis vertical transition of...

Hi,

Check your experimental UV-Vis. spectra, select those bands (excited states) which are comparable (very close) to the simulated spectra and also for which bands oscillator strength vales are maximum.

It depends also on which parameter you want to highlight. Are you interested in the optical gap? Then it should coincide with the 1st excited state (ES), provided it has a non-negligible oscillator strength. Do you want to stress the maximum possible absorption? Then you will highlight the ES with the largest intensity. Or, if you are interested in the absorption spectrum of the material, you will visualize all the calculated ES with a program like Multiwfn that will produce a curve from the ES peaks.

https://www.researchgate.net/profile/Muhammad-El-Saba

Dear muhammad-El-Saba

The complex is a schiff_base macroacyclic complex.

The max lambda at the first excited is not jn a good position and it differs from experimental uv. I dont know if i can report the second or third excited state instead or not. Ive sent a picture here

Dear Corentin Lefebvre,

Thanks for your answer.

I performed the calculations in gaussian09 at b3lyp/3_21 g*, since the structure is large and the caculations take alot of time.

I tried to install multiWFN but each time a black dos screen with an error appears.

My goal is to compare experimental with theoretical results.

But the max lambda at first excited state is not in good agreement with the experimental.

Dear Mohammed Enamullah,

Thank you very much.

The sixth excited state in output file is in good agreement with the experimdntal. Can I use the data of the sixth excited state?

This is the data

Dear Moly Crist ,

I can understand the difficulty to perform this kind of calculation and the necessary amount of time that is needed. But, if you don't locate your singlet excited state in common with the experimental one, it might be your functionnal (B3LYP) is not appropriate and/or your basis-set (which is very small and certainly not good for your calculation). Generally, a range separated hybrid functional is used (CAM-B3LYP) with a good amount of HF E_{exc} (wB97XD for example). For the basis-set, a large one is advised to correctly simulate charge transfer, among others. I hope this could help you !

Dear Corentin Lefebvre,

Thank you very much for your helping me. As you mentioned, I should perform the calculations with a larger basis set.. I'll try it as soon as possible.

Dear Moly Crist ,

Do you perform this calculation with your own computer ? I could understand the difficulties you're facing, especially studying a so heavy molecular system like yours. Do you have an access to a supercomputer and other QM program like ORCA that are more appropriate for performing excited-states calculations ?

Dear Corentin Lefebvre,

Right now I don't have any access to the suppercomputer. But I can make it, If my university cooperates