The vertical excitation involves transition from the ground state to the excited state without any structural rearrangement. The energy gap will be somewhat larger than that for adiabatic excitation, where the excited state geometry is allowed to relax, but is useful to know since you can often expect large Franck-Condon factors and therefore healthy absorption cross sections for the vertical excitation.

So, to determine the vertical absorption energy you need the electronic energy of the ground state with optimized geometry, and you need the electronic energy at the same level of theory for the excited state of interest, using the previously obtained ground state geometry (ie this is from your single point tddft calculation). The absorption energy will be the difference.

Just make sure you are using an optimized ground state structure at the same level of theory, and that you have identified the correct excited state of interest, and the single point tddft calculation should give you the energy you are after (whether or not it is accurate is another question entirely).

The vertical excitation involves transition from the ground state to the excited state without any structural rearrangement. The energy gap will be somewhat larger than that for adiabatic excitation, where the excited state geometry is allowed to relax, but is useful to know since you can often expect large Franck-Condon factors and therefore healthy absorption cross sections for the vertical excitation.

So, to determine the vertical absorption energy you need the electronic energy of the ground state with optimized geometry, and you need the electronic energy at the same level of theory for the excited state of interest, using the previously obtained ground state geometry (ie this is from your single point tddft calculation). The absorption energy will be the difference.

Just make sure you are using an optimized ground state structure at the same level of theory, and that you have identified the correct excited state of interest, and the single point tddft calculation should give you the energy you are after (whether or not it is accurate is another question entirely).

Ok, sounds like you are comparing the energy of the ground state with itself, which should be zero. There will be some information on the excited states in the calculation output (format depends on what code you are using). It is probably quoted as an absorption energy in nm from the ground state, for some number of lowest lying excited states.

Ok, Yes, i guess that vertical absorption energy is in the output of single point TdDFT calculation. i calculated TddfT with gaussian and i know its output file. i want to know if first excitation energy is vertical absorption energy?

The first vertical absorption energy means S0 -> S1 absorption energy

Dear Samaneh,

It can vary.If you are calculating HOMO to LUMO excitation energy then you should first find out what is the orbital number for HOMO and LUMO. Then check which excitation has maximum contribution for the transition HOMO->LUMO. In your TDDFT output there will be several transition for each excited state and the number right side of that excitation represent its contribution. So, if you have calculated reasonable number of excited states, one excited state should have large contribution for your desired transition. Excitation energy for that excited state is your vertical excitation energy for your desired excited state.

thanks Dear Soumen

Usually, the first excitation energy, has the largest contribution for the transition HOMO to LUMO !

Dear All,

May i join the discussion with a question, the lowest energy HOMO->LUMO transition calculated from TD-DFT always has energy lower than the HOMO-LUMO energy gap (Elumo-Ehomo, same solvent effect already). Do you have any suggesting how to explain that.

Thank you!

you can sear the word " Excited State  " in output file

one example:

Excitation energies and oscillator strengths:

Excited State 1: Singlet-SGU 16.8484 eV 73.59 nm f=0.5855 =0.000

1 -> 2 0.70770

This state for optimization and/or second-order correction.

Copying the excited state density for this state as the 1-particle RhoCI density.

Excited State 2: Singlet-SGG 31.3161 eV 39.59 nm f=0.0000 =0.000

1 -> 3 0.70849

Excited State 3: Singlet-SGU 51.3232 eV 24.16 nm f=0.1000 =0.000

1 -> 4 0.70674

keep in your mind that the first Excited State energy will be lower than HOMO-LUMO energy gap. it is called optical gap

Hello, if you have calculated reasonable number of excited states from Gaussian results, one excited state should have large contribution for your desired transition.

Excitation energy for that excited state is your vertical excitation energy for your desired excited state.

Thanks for Dr. Soumen Ghosh

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