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Dear Tymofii and Safiya,
Yes, Tymofii is right about Kohn–Sham orbitals. We can even recall that the variational principle does not hold for approximate functionals... But are HF orbitals required to be better if the electron correlation is not accounted for in the computation? And electrons are again moving in some averaged potential of their counterparts, like in KS-DFT. I am not a real computational chemist, but as far as I can judge, there are quite severe approximations in both cases, although whereas HF and PT are continuously improving the model by adding new terms to it, KS-DFT uses alternative approach of building the structure "from scratch". There is also continuous evidence
that the form of KS-DFT orbitals is usually quite rigorous
(see classical paper on this topic by Stowasser and Hoffmann,
1999 http://dx.doi.org/10.1021/ja9826892,
Chong, Gritsenko & Baerends, 2002 http://dx.doi.org/10.1063/1.1430255
– several-small-molecules study that concludes B3LYP (sic!)
orbitals are closer to Dyson orbitals than HF ones). If I do
not mess something up, what KS-DFT really can have problems
with are degenerate states and, of course, multireference
wavefunctions.
As Safiya is studying
physisorption, electron correlation can be quite important for
description of the system (due to non-covalent interactions).
Could this be the reason of HF and B3LYP orbital shape
differences? Sincerely, I am just making conjectures. With best regards, Igors Mihailovs engineer/PhD student ISSP, University of Latvia On 18.10.2016 00:12, Tymofii
Nikolaienko tim_mail-,-ukr.net wrote:
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