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I have done the geometry optimization with the B3LYP functional, however, there is a need to calculate the excited state energy, so the CAM-B3LYP functional should be better suited for that. Do I need to reoptimize the geometry to use CAM-B3LYP or are these functionals simillar enough so no reoptimization is needed?

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    $\begingroup$ Assuming you have enough time, you could probably try it out anyway and see whether you get much of a difference. Otherwise, when it’s time for you to present your work, somebody will ask about it anyway... $\endgroup$ – orthocresol Dec 16 '20 at 13:18
  • $\begingroup$ @orthocresol, I have seen in a couple of articles, which model really similar molecules, that first they optimize with B3LYP and then calculate excited states with CAM-B3LYP or even other functionals and compare the results, so I was wondering, if it is really necessary. $\endgroup$ – aerospace Dec 16 '20 at 13:33
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What is sometimes done is use the geometry and thermochemical corrections (i.e. the results of the frequency calculation) of method X, but use the electronic energy (SCF energy) of another method Y. This is generally noted Y//X. Since the geometries and thermochemical corrections typically are not much influenced by the quality of the method (beyond a certain point), this scheme gives good result. The electronic energy can however benefit from a larger basis set or a better computational method (e.g. MP2 with DFT geometries).

I am assuming this is what you have seen in the literature, as it is relatively common, especially for large molecules like organometallic complexes. I am not too familiar with excited states, but the same thing likely applies.

However, you must always use the exact same method for the geometry optimisation and the frequency calculation. The frequency calculation uses the curvature of the potential energy surface (PES) to calculate approximate corrections. If you are not at an extremum (minimum for equilibrium structures or saddle point for transition state), the results are meaningless. Changing the method will move you off the extremum sufficiently and your frequency calculation will likely have several imaginary frequencies.

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  • $\begingroup$ Thank you very much for the comprehensive answer. Yes, I am actually doing calculations for a large organometallic complex, however, I am planning to still to use the DFT method for calculations and just to change functional of it for energy calculations (and the same was done in the articles). Does the following still apply? $\endgroup$ – aerospace Dec 16 '20 at 14:58
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    $\begingroup$ Yes, you can use the SCF energy of a different DFT functional as well $\endgroup$ – Raphaël Dec 16 '20 at 18:04
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    $\begingroup$ Excellent points! I'd like to add that in most cases, you should probably do a calibration of different functionals, see here. You might also find this question on Matter Modeling SE helpful. @aerospace $\endgroup$ – Martin - マーチン Dec 16 '20 at 18:36
  • $\begingroup$ @Martin-マーチン, thank you for the recommendations! $\endgroup$ – aerospace Dec 16 '20 at 20:15

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