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VPT2 calculation using TDDFT

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Member
Registered: Nov 2016
Posts: 2
I have used Q-Chem successfully to obtain fundamental frequencies (using DFT, including VPT2 anharmonic corrections) for closed-shell molecules in their electronic ground states. However, I run into problems when attempting to get anharmonic corrections for the S1 excited state using TDDFT [for example, formaldehyde]. The harmonic frequency calculation comes out fine and gives normal-mode frequencies close to experimental values. But the fundamentals calculated by VPT2 and TOSH are unphysically large (on the order of 10,000 cm-1) -- even for modes where Fermi resonances are not expected to be a problem.

Does anyone know if Q-Chem has limitations in its implementation of VPT2 for excited states?
Member
Registered: Sep 2007
Posts: 1
Without seeing the input, my best guess is that the problem is due to poor interplay between the excited state calculation and finite difference code used to compute the 3rd and 4th derivatives. I suspect the ground state energy and gradient is being written to file and these are being combined with the excited state Hessian to generate the incorrect results. I should be able to confirm this if I can see the input being used.
Member
Registered: Nov 2016
Posts: 2
Thank you for the information. Ultimately I found out I had set the input parameters correctly. I had neglected to include
ideriv 1
When this is included, the TDDFT/VPT2 calculation comes out fine.

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iOpenShell » Q-Chem specific questions » VPT2 calculation using TDDFT