[Pw_forum] what is the relevance between the Kohn-Sham Orbital isosurface and real molecular orbitals?

[Ari P Seitsonen]

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
>