CCL:G: HOMO-LUMO molecular orbitals



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:

Sent to CCL by: Tymofii Nikolaienko [tim_mail##ukr.net]
Dear Safiya Amer,

sorry for saying such not-popular thing, but it is a big question whether any orbitals obtained from DFT do
have any physical or chemical sense.
Just recall that most of the widely DFT functionals are based on Kohn-Sham formalist - that is, the orbitals
you get are the orbitals for 'auxiliary' system of non-interacting electrons and, strictly speaking, the only
linkage between this system and your real system is that they have the same electron density.
However, having the same density does not imply having the set of the same orbitals.
Thus, it you might want to consider an alternative of just ignoring the shapes of DFT orbitals.

Best regards,
Tymofii



On 16.10.2016 22:01, Safiya Ess Amer amersaf85%%yahoo.com wrote:
Sent to CCL by: "Safiya Ess Amer" [amersaf85 ~ yahoo.com]
Hello for all CCL subscribers
I'm studying a system of hydrogen molecule adsorbed on an ionic surface. I
used Gaussian 03W programme with two methods HF and DFT-b3lyp to obtain
molecular orbitals by using GaussView programme. this rout section is used for
that:

%Chk=Th90 b3lyp
#T b3lyp nosymm  gen pseudo=read pop=full

  Now I'm looking forward to explain the difference between what I got by using
HF and DFT. in other words, I got a certain shape of molecular orbitals using
HF and other different shape using DFT, how can I explain this difference!

Thanks in advance


Safiya Amer
Graduate student
Misrata University
amersaf85^yahoo.com>