In using Opt TD=(nstates=n) in Gaussian, I get many data points for excited state energies of each state, i.e. the results for the TD calculation are repeated after each optimization. Should I just choose the last set? The energies are quite different for the same # excited state each time (~70nm difference for excited state 1 across the repeated results).
If you try use both keywords at the same time (TD and OPT), the software will do the TD approximation calculation on every step of the optimization. I guess that is not what you are looking for.
If you want to do TDDFT on an optimized structure, you should do the OPT without the TD keyword first. After that you use the optimized structure in a single point calculation with the TD keyword.
It is just as simple as doing a two-step process instead of trying to do it in one calculation.
Just to complement: If you already have an OPT + TD calculation done, you'll see that each set of excited states is associated with one geometry of the OPT run, so the TD over the optimized structure will be the last one printed. I.e. the one associated with the optimized structure. Just be sure that you are reading the set that is related to the geometry you are analysing (be it the optimized one or not).
Lucas Gian Fachini How would you go about the two step process? I tried to use link1 and do a second calculation line after the coordinates and read from a %chk file and I kept getting segmentation errors among other errors. Can you give an input example of doing an Opt then TD? Thank you for your help!
For most molecules the easiest way is doing an OPT calculation, getting the geometry from the optimized molecule, putting it in a new input file and running another job with the new geometry (without using OPT as a keyword). This will do a full calculation with default SCF convergence criterion for TDDFT on the desired optimized geometry.
Reading the .chk for a guess is useful if your system is very large, so you can read the guess orbitals from the .chk file and cut some steps of the calculations, but to do this I'd advise you to read more info on the gaussian keywords site: https://gaussian.com/guess/
The keywords for reading the .chk file are usually Guess=Read (for the guess orbitals) and Geom=Checkpoint (to read the geometry from the .chk).
Lastly, if you plan to do it all in a single input file (multi-step job) I'd advise you to read the input section of the manual, as it will have the correct syntax for your usage. https://gaussian.com/input/
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