[Pw_forum] Small U for Co in ZnO for LDA+U (update)

[Nektarios Lathiotakis]

Hi again,

Mateo is right,  the occupations for Co are 0.4 instead of 1.
A last question that is a bit different...

The gap of pure ZnO (no Co impurities) can not be fixed by pure lda+u
because the Zn-d states are fully occupied. However, (I guess) the gap must
be sensitive to a hubbard U for oxygen p-states and could be fixed by
fitting U_oxygen.
However, I am not aware of anyone using Hubbard U for p states... and a
quick search
in bibliography didnt find anything.

On the other hand Oxygen is in elements list in tabd.f90.  That makes me
suspect that 1) It might not
be completely non sense 2) there might be other attempts to do that. Any
comment?

Nektarios

On Mon, Jun 30, 2008 at 8:53 PM, Matteo Cococcioni <matteo at umn.edu> wrote:

>
> Dear Nektarios,
>
> I was going to write but Nicola has been faster than me. Nicola is right:
> probably the reason why
> "ortho-atomic" and "norm-atomic" produce similar results is that the atomic
> wavefunctions in the Co PP
> are not normalized. Try to check the occupations in the "atomic" case. are
> they much bigger than 1?
>
> Ortho-atomic: atomic wfcs are orthogonalized with those belonging to
> different atoms.
> atomic: wfcs are orthogonal only to those belonging to the same atom (there
> are overlap regions in between atoms)
> norm-atomic: quick and dirty fix added to use PPs whose wfcs are not
> normalized. when these wfcs are normalized it gives the same results as
> "atomic".
>
> In general I would suggest to not look at the value of U itself but rather
> to the results it produces. I just found a paper: PRL 100, 256401 (2008)
> where they use U=2 for Co and 5eV for Zn (with Vasp). Why don't you
> actually try to compute it (PRB 71, 35105 (2005))?
>
> hope this helps.
>
> Matteo
>
>
> Nektarios Lathiotakis wrote:
>
>> Hi again,
>>
>> As I wrote previously I have a Co impurity atom in ZnO and
>> trying to find the right Hubbard-U for Co by fitting to experimental Co-d
>> projected band  from J. Appl. Phys. 103 07D130 (2008). A typical input
>> file is in my
>> previous e-mail.
>>
>> I played with U_projection_type parameter and:
>>
>> 1) 'ortho-atomic' and 'norm-atomic' agree and give the same optimal U for
>> Co
>> that is around 4eV. Very reasonable value and no referee would ever
>> object.
>>
>> 2) 'atomic' (the default) gives as optimal U 0.5eV which is much smaller.
>>
>> In other words the Co-d projected band you get with
>> U_projection_type='ortho-atomic'
>> and U_Co = 4 eV is the very close to the band you get with
>> U_projection_type='atomic'
>> and U_Co = 0.5 eV.
>>
>> Why is that difference? The problem with changing the U_projection_type is
>> that only
>> atomic supports forces.
>>
>> Thanks in advance and also to Nicola for his previous reply.
>> Nektarios
>>
>>
>>
>> --
>> --------------------------------------------------------------------
>> Dr. Nektarios N. Lathiotakis
>> --------------------------------------------------------------------
>> Theoretical and Physical Chemistry Institute
>> National Hellenic Research Foundation
>> Vass. Constantinou 48, GR-11635,
>> Athens, Greece
>> --------------------------------------------------------------------
>> Tel: +30 210 7273805 FAX: +30 210 7273794
>> Inst. secretary: +30 210 7273792, +30 210 7273795
>> --------------------------------------------------------------------
>> Web: http:www.eie.gr/nhrf/institutes/tpci/cvs/cv-lathiotakis-en.html <
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-- 
--------------------------------------------------------------------
Dr. Nektarios N. Lathiotakis
--------------------------------------------------------------------
Theoretical and Physical Chemistry Institute
National Hellenic Research Foundation
Vass. Constantinou 48, GR-11635,
Athens, Greece
--------------------------------------------------------------------
Tel: +30 210 7273805 FAX: +30 210 7273794
Inst. secretary: +30 210 7273792, +30 210 7273795
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Web: http:www.eie.gr/nhrf/institutes/tpci/cvs/cv-lathiotakis-en.html
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