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

[Matteo Cococcioni]

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
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> Athens, Greece
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