Hi,
I have been trying to calculate the frequency vibrations of the excited state S1 of an organic molecule (24 atoms). The geometry optimisation of the excited state converged smoothly howerver the energy oscilates back and forth between 10-2 and 10-3 when I run the frequency calculation. I am not using a very large basis set (cc-pVTZ). I am very thankful if someone can help with any suggestion.
Best
Dear Salma Bejaoui,
Maybe what you get is not a minimum, but a transition state (I had that problem working with excited states).
First of all, which software are you using? Maybe is a problem with (lack of) analytic second derivatives for excited states in your code. If you are computing them numerically... Well, it could explain the oscillations in energy.
Anyway, some tests you can do, let's see if some of them works!
-From the optimized structure, optimize again (maybe you got some crossing between the states and you are not in the actual S1 after the optimization).
-You can also move a bit some atomic position before restarting the optimization (maybe although you thought it was a minimum it is, in fact, a transition state... Chemistry of excited states can be really surprising!)
-Also, you can disable symmetry. Maybe it will help.
-If second derivatives are being computed numerically, increase the accuracy in the optimization and reduce the step in the frequency calculation.
-You can do these tests using double-zeta basis set, as cc-pVDZ. Geometries provided by double and triple-zeta are very similar and also you can see the effect of the basis.
Hope it helps,
Fernando
Dear Salma,
next time, please specify which program you are using as this would really help to identify the problem.
However, I'm pretty sure I can guess what you are seeing and it is good news: The oscillations in the energy are the results of the finite displacements your program uses to calculate the Hessian, since it presumably does not have analytical second derivative for TDDFT implemented.
So just wait for the calculation to finish (there should be 3N*2 so 144 displacements required, i.e., every atom in 3 dimensions plus and minus).
Hope that helped.
Jan
Hi,
Thank you for your answers.
I am using Qchem. The Hessian should be calculated analytically with TDDFT+B3LYP. When I used cc-pVDZ as suggested by Fernando, I got the S-1 frequencies but many of them were imaginary. Those complex frequencies disappeared when I disable the symmetry.
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