If there was a deviation between the experimentally calculated values and theoretical determination done by DFT calculation, then what is the reason?

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    $\begingroup$ I would say this question is too broad. The answer clearly depends a lot on (1) whether the deviation is significantly larger than the typical error of that functional, and (2) which deviation you are talking about, e.g. do you mean a deviation of the formation energy, or a deviation of the band structure, or a deviation of the lattice constants? $\endgroup$
    – wzkchem5
    Jan 4, 2022 at 12:12
  • $\begingroup$ The question can stay open if you take wzkchem5's advice, I've also adjusted the body to match the title. $\endgroup$ Jan 4, 2022 at 18:16
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    $\begingroup$ Unless this question is heavily narrowed, I do not think it can remain open. This is far too vague and potentially already answered in other questions. $\endgroup$ Jan 4, 2022 at 21:37
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    $\begingroup$ There are several questions already dealing with issues in DFT, I know because I made one of them mattermodeling.stackexchange.com/q/526/181 $\endgroup$
    – B. Kelly
    Jan 4, 2022 at 23:06
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    $\begingroup$ I agree this is too broad. As a start, consider: 1) Is it a ground state property - i.e. would "the true" functional predict it correctly? 2) Is it a failure of our approximate functionals? 3) Do we need finite temperature simulations? 4) Is it a failure of practical DFT implementations, e.g. Born-Oppenheimer approximation? 5) Is the actual system modelled the same as the experimental one (e.g. are there defects in the experiment)? 6) Is it because we're pretending Kohn-Sham states are electronic/quasi-particle states, instead of actually using a constructed functional? $\endgroup$ Jan 4, 2022 at 23:16