I am working on a project where I have to calculate the TDDFT excitation energies of organic molecules (to compare to the experimental UV/visible i.e. optical absorption spectrum). Now, if I understand correctly, most of the time, the absorption peaks in the UV/vis spectrum correspond to a vertical excitation i.e., the geometry of the molecule stays the same while the electron density shifts quickly (Frank-Condon).
The vertical excitation (blue line in picture) happens from the lowest point of the PES of the ground electronic state($E_0$), i.e. from the minimum energy geometry. This seems to suggest that excitation energies (with TDDFT or any other method) should be calculated only using the method/basis set combination in which the geometry is a minimum. So, I should optimise the geometry using the same method that I am using for excitation energies.
However, I have seen research papers where EOM-CCSD excitation energies were calculated on geometries obtained with B3LYP/6-31G* for example. Is this correct? Is there any error in the calculated values if I use different functionals or basis sets for the geometry optimisation and the excited state calculation?