Recently, in a research paper, i found that the author had optimized the structures in an S1 excited state (S0 = G.S).
Greetings, regarding the syntax, It is really easy.
In Gaussian the keyword "root", used in TD, select a given excited state to be optimized, so, let's consider you want to optimize the third excited state:
#p opt TD=(nstates=6,root=3) wb97xd/def2tzvp
This will calculate 6 excited states and optimize the third one (using the theory level you choose, for example, wb97xd/def2tzvp). If you already have performed a TDDFT calculation before and don't want to recalculate everything, you can read the results from the .chk file:
#p opt TD=(read, root=3) wb97xd/def2tzvp density=current guess=read geom=allcheck
But ALWAYS select a couple or more excited states on top of the one you are optimizing, otherwise you won't get proper results, for example, for root=3 try to always select at least nstate=6.
You can also implement Tamm-Dancoff aprroximation to speed up the optimization in TDDFT route.
Hello Manik Das
As other friends answered, you can use GaussView software to create the input you want and then perform the optimization process on it in Gaussian software.
A point that is very important, Gaussian calculations are usually heavy and its implementation on personal computers and laptops can be a very time-consuming process. On the other hand, it may damage your system a lot. MolQube company specializes in Gaussian calculations. According to your needs, CPU and RAM will be assigned to you and calculations will be performed for you. If you encounter an error, they will give you advice on how to fix it.
Their website address is www.MolQube.com and their email address is [email protected]
Good luck
Hello Manik Das
The answer provided by Lucas Gian Fachini is excellent also, the suggestion of Rohan Sharma to use the Tamm-Dancoff approximation is promising to speed up your calculation.
I want to state a few points. First, you should see the authors' methods and then try to understand why they chose that particular method. Second, I know that you have recently started doing calculations, so you should review the Gaussian website documentation.
Link 1: https://gaussian.com/td/ This may help you with the input and also with the literature to understand.
Link 2: https://www.youtube.com/@dbinfotech/playlists These videos may provide many different types of runs.
On another aspect, I'd suggest you read the articles on the DFT and TDDFT methods benchmarks, which will provide you with more details.
I hope this helps.
Dear Manik Das,
If you are using gaussian16, instead of using TD(nstates=...) use TDA(nstates=..) which invoke TDA approximation.
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