Dear Yavar Taghipour Azar,
From what I know, only the energy eigenvalue HOMO should have a physical
meaning - the ionisation energy - but also it would naturally require the
usage of the exact exchange-correlation functional. All the rest relies on
the empirism, that they seem to correspond quite reasonably the
qualitative, often also quantitative results from the experiments and
chemical picture/intuition, or the more accurate theoretical methods
(quantum chemical approach like coupled-cluster methods, many-body methods
like GW and Bethe-Salpeter equations). So there is no theoretical
foundation for the usage of the Kohn-Sham orbitals and eigenvalues, just
the overall general agreement and convenience (which can sometimes be read
also as "lazyness" ;).
Hopefully this clarifies your confusion at least partially. The
textbooks should discuss this also this more "practical" aspect, strange
if they don't. I don't remember when/if this topic has been discussed
already before (mailing list/FAQ), sorry.
Greetings from Zurich,
apsi
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
Ari P Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
Physikalisch-Chemisches Institut der Universitaet Zuerich
Tel: +41 44 63 54 497 / Mobile: +41 79 71 90 935
On Sun, 30 Dec 2012, Yavar Taghipour Azar wrote:
>
> ?Dear QE user
> ?
> As mentioned in some text books (for eample, A Chemist?s Guide to Density
> Functional Theory);
> The orbitals satisfying Kohn-Sham equation? have no physical significance
> and their only connection
> to the real world is that the sum of their squares add up to the exact
> density.
> ?
> by considering this, I cant understand physical relevance of isosurfaces
> which plotted in pw postprocess and
> the real molecular orbitals (HOMO, LUMO, LUMO+1................).
> ?
> thanks in advance
> ?
> Yavar Taghipour Azar
> ?
> Condensed Matter Section, AEOI
> ?
> Tehran, Iran
> ?
