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Alex Granovsky
gran@classic.chem.msu.su

On Tue Jul 1 '14 1:23pm, Siddheshwar Chopra wrote
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>Dear All,
>I need to know: If I wish to compare TDDFT calculations from different functionals like B3LYP, PBE0 etc., then is it necessary that I MUST run the optimization/hessian with the SAME functional?

Strictly speaking it is not necessary to use the same functionals.
Assume functional A can predict equilibrium geometries
better than functional B while excitation energies can be better
with B. Then it would be logical to use A for ground state
geometries and B for excitation energies.

>Also, if I optimize with say 6-31G basis set, then can I compare the 6-31G and 6-31G (d,p) TDDFT results using the same optimized structure from 6-31G?

Once again, this is not absolutely necessary but most people prefer
to use the same basis sets.

Kind regards,
Alex Granovsky