[Pw_forum] Doubt in magnetic behaviour

[Giuseppe Mattioli]

> While performing scf calculations for a
> semiconductor to which a transition metal has been added,  which
> occupations should be used, fixed or smearing?

Your question doesn't have an easy answer.
I'll try to share my (little and quite empirical) knowledge.
If you add some gaussian smearing (a value of 0.01 Ry or less) your 
calculations will converge faster, but your electron density could be 
"delocalized" amongst Kohn-Sham wavefunction, the eigenvalues of which are 
placed across the Fermi energy of your system. This is particularly true when 
smearing value is too large wrt the energy difference between occupied and 
unoccupied bands, e.g., when your dopant atom induce rather shallow levels in 
the semiconductor band structure.
For example, you could find that an occupied dopant level is "pinned" to the 
conduction band because of the smearing contribution, i.e., its K-S eigenvalue 
position and the total energy of your system depends upon the smearing value;
On the other hand, such systems hardly converge if you use fixed occupations. 
At every electronic step your system starts to dance between, e.g., a 
configuration in which one electron is located on the impurity and another 
configuration in which the electron is located on the conduction band minimum.
This is very painful when you have a molecule placed above (i.e., non 
interacting with) a semiconductor surface...
So, as usual, there is no free lunch... Make several tests. I can only say 
that I always try to use smearing occupations, because of the convergence 
issue. But look always at the smearing contribution to the total energy. If it 
affects heavily the total energy value, then don't bet too much money on your 
results...:-)

Further comments and suggestions are welcome...

Yours

Giuseppe


On Friday 15 May 2009 15:32:08 Padmaja Patnaik wrote:
> Thanks for your suggestion.
>
> I have another querry, while performing scf calculations for a
> semiconductor to which a transition metal has been added,  which
> occupations should be used, fixed or smearing?
>
> Thanks in advance
> Regards
>
>
> Padmaja Patnaik
>
> Research Scholar
>
> Dept of Physics
>
> IIT Bombay
>
> Mumbai, India

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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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