Is it rigorous enough to change a DFT functional after geometry optimization to calculate excited state properties?

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?

• 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... – orthocresol Dec 16 '20 at 13:18
• @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. – aerospace Dec 16 '20 at 13:33

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).