# How to assign UV-vis maximum absorption wavelength theoretically by using td-DFT?

I'm currently working on the simulation of Uv-vis absorption spectra of some organic compounds, while performing TD-Dft the log file that we got contains the values of deltaE of the transition,wavelength, oscillatior strength f and the electronic contribution %

for exemple

 Excited State   1:      Singlet-A      2.8918 eV  428.74 nm  f=0.4068  <S**2>=0.000
71 -> 72         0.70317


I got confused with all these parameters for choosing the londa max, from what I've known the oscillator strength f : is the probability of a transition to occur , but i've found that the maximum value of f doesn't necessary involve the maximum londa ,

even for the value in ev deltaE bigger-xx->londa max bigger ,

can someone help me figure this out, thank you

• @Alme999. Maybe I do not understand your question but perhaps you are misinterpreting the information you got from the Gaussian output. The output you are writing correspond to a vertical transition (no allowance for Franck-Condon factors), this is the difference in electronic energy between the molecular configuration in the ground and the selected excited state. This value maybe close to the maximum of your absorption spectrum. In that case the value of $\lambda_{max} = 428.74 nm$.
– PAEP
Mar 8, 2023 at 10:58
• The oscillator strength $f$ is related to the probability of the electronic transition not to the position of the maximum of the spectrum. Maybe you find useful the information on the topic on Gaussian web page gaussian.com/uvvisplot
– PAEP
Mar 8, 2023 at 10:58

There are different ways to know the lambda max. You can visualize the output by using different programs, and you will find that there are different bands or only one band depending on the system. Each band is made up of the highest wavelength with the highest oscillator strength (this band is a transition between orbitals).

For example, Molden. As you can see in the attached image. There are two bands, one with a wavelength of 180 nm and another with a wavelength of 230 nm. Therefore, these two bands refer to the maximum wavelength with the highest oscillator strength. Note:: you can visualize the molecular orbital.

There are other programs like Avogadro, you can plot the spectrum easily. Another program Chemissian, it is not free, but in this program, you can visualize NTO orbitals.

• I'm using gaussview but I still can't determine the maximum londa precisely , so I tried to visualize my spectrum in another program. the maximum wavelength that I've found is different from those that are displayed in the output file of gaussian , for example londamax=273.00('from visualization') , in the output there are values such as 267 280 270, and I want to determine the electronic transition contribution ETC% and its oscillator strength. should I use the closest wavelength among them all in this case it's 270 nm right? Mar 4, 2023 at 12:35
• @AIme999. You can do that. But you should check the value of oscillator strength as well. 270 nm should have the highest oscillator (f). Mar 4, 2023 at 13:55
• What is the molecule depicted in this graph? Jul 25, 2023 at 18:02
• @ksousa . It is boron complex, why? Jan 6 at 18:03