Hi Suvendu Paul!

If you intend to use the DFT formalism then you should follow the next steps:

1. Optimize your molecule in its ground state (#   ... opt freq PBE1PBE/cc-PVTZ ...). Be sure that you are on a minimum on the PES, i.e., all the frequencies are positive.

2. Calculate the absorption spectrum using the TD-DFT formalism (# ... TD(nstates=30) PBE1PBE/ccpVTZ ....). For this step, use the previously optimized geometry as starting geometry .

3. Identify the excited state of interest (let's suppose it is the state 1) and optimized it (#  .... Opt TD(root=1, nstates=5) PBE1PBE/cc-pVTZ ....). The starting geometry for this step could be the optimized ground state geometry, slightly distorted.

4. Calculated the vertical emission energy as the difference between the energy of the excited state of interest (you can find it in the output file obtained in step 3 by searching back from the end of file for the string "TD-KS)") and the ground state energy of the excited state geometry (find it in the output file obtained in step 3 by searching back from the end of file for the string "HF=-").

Good luck!

Vasile

Dear Chis,

First of all thanks for your response. But, I know the process to calculate vertical excitation and vertical emission energy. My target is something different. I want to generate fluorescence emission spectra theoretically as stated in SCRF keyword by Gaussian. I observed so many people to present the theoretical spectra in their publication.

If you need the vibrationally resolved absorption and emission spectra in gas-phase, then the required steps are given in the attached pdf file (Vibronic structure ...). We followed the procedure to obtain the data reported in the attached paper (Absorption spectra of PTCDI: ...). If you need to include the solvent effects then follow the procedure given in the Examples section at http://gaussian.com/scrf/

Good luck!

Vasile

http://gaussian.com/scrf/

Article Absorption spectra of PTCDI: a combined UV-Vis and TD-DFT study

Dear Dr Chis. Your answer is correct. Is it possible to sent me details

for the shape of fluorescence in the presence of solvent ?

Dear Suvendu Paul,

Have you calculated and plotted emission spectra ? Can you please tell how does calculate and visualize emission spectra by Gaussian and GaussView ?

Thank You

Shilendra