|
Hm... To be honest,
"The nature of the chemical bond—1990: There are no such
things as orbitals!" [ J. F. Ogilvie, J. Chem. Educ., 1990,
67 (4), p 280, DOI: 10.1021/ed067p280 ,
http://pubs.acs.org/doi/abs/10.1021/ed067p280 -- a rather
long-lasting discussions for which I can provide more references,
both
'pros' and 'cons' if necessary; of course, notwithstanding this
discussions I do use the concept of orbitals myself - but mostly
in order to
understand my system in a 'chemical' terms (and get some
heuristics), but perhaps _not_ in order to explain why certain
interaction/bonding occurs ]
:)
Going seriously, my statement was merely that in Hartree-Fock
(where all limitations of this theory are known in advance) we
get (canonical) orbitals as eigenfunctions of the one-particle
Fock operator and we can 'track' a clear and direct relationship
of
this operator to the real physical system under study. In contrast
to this, in Kohn-Sham DFT we get the orbitals as eigenfunctions
of an operator which was set-up for a 'completely' different
system (a Kohn-Sham system) which is a priori (!) a different
system.
(note that in DFT we do know the electron density of the system
but we never know the wavefunction!)
Of course, I realize that in practice all these fundamental
distinctions are often quite small (if not negligible), but
nonetheless
it's a pity that often researchers simply forget/ignore the fact
that these distinctions exist -- for example, when a
2-nd order perturbational analysis within the NBO method is
applied to the 'density matrix' obtained from DFT.
Best regards,
Tymofii
On 18.10.2016 17:12, Igors Mihailovs
igorsm[]cfi.lu.lv wrote:
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>
|