[Pw_forum] Small U for Co in ZnO for LDA+U

[Nektarios Lathiotakis]

Hi all,

the option 'atomic' is the default and maybe that should change? I dont
know. It created
me a lot of trouble however. I confirm Shu-jun numbers (2.6 occupations)...
I get the same.
I think we used the same functional and consequently potentials for
Zn, O, Co (PBE functional).

Shu-jun, did you use the same pseudo for Co in ZnO and SnO2?

With the 'atomic' option  in this case , U_Co=0.5 is enough to push all
Co-d states into the valence. The rule I found is that the U_Co with option
'atomic' has the same effect as 8*U with option 'ortho-atomic' on the Co-d
projected DOS.

So the self consistent value of Shu-jun correspond to U=2.5eV with
ortho-atomic. Of course this applies to
the position of Co-d occupied bands. Dont know about other properties. I
will have to
repeat calculations with 'ortho-atomic' or 'norm-atomic' option.

Thanks for the discussion
Nektarios

On Thu, Jul 3, 2008 at 10:16 AM, Nicola Marzari <marzari at mit.edu> wrote:

>
>
>
> Dear Shu-jun, All,
>
>
> I get very nervous about using U with non-normalized occupations. Even
> if U can have some amount of pseudopotential dependency, using it
> with unnormalized projectors really messes up any meaningful discussion
> of occupation values and U values.
>
> And of course referees would go nuts at seeing that a U=0.3eV is
> relevant - the reason is that that number doesn't really make sense.
>
> To me, it's an option ("atomic") that should be removed altogether from
> the code - others might have other opinions, but if we leave it as it
> stands, we risk creating an enormous amount of headaches down the line.
>
>                        nicola
>
>
>
>
> ??? Shu-jun Hu wrote:
> > Dear All,
> >
> > I would post my comments on the topic of LDA+U calculation of small U_Co.
> >
> > (1) The self-consistently calculated U_Co is indeed sensitive to the
> oxidation
> > state of Co. In Co-doped ZnO DMS, the value is about 0.31 eV, a very
> "small"
> > number. However, in Co-doped SnO2, we got a "reasonable" value of 3.7 eV.
> > U_projection_type is specified as the default (atomic) in both cases
> during the
> > calculation of U_Co.
> >
> > (2) The calculated band structure is much sensitive to the value of U_Co
> and
> > U_Co=0.31 eV does have a significant effect with respect to U=0.
> U_Co=0.31 eV can
> > well fit the experimental results (APL 84 4233 (2004), and also J. Appl.
> Phys. 103
> > 07D130 (2008)), where the occupied Co:3d majority state are deep in the
> valence
> > band rather than in the gap. If U_Co=2 to 5 eV is implemented, Nektarios
> is right:
> > a Co-d projected DOS far deep into the valence (completely non sense).
> >
> > (3) The occupation is larger than 1 but not less than 1 in the "atomic"
> case. A
> > piece of output file is listed in the end.
> >
> >     Best wishes.
>
>
>
>
> --
> ---------------------------------------------------------------------
> Prof Nicola Marzari   Department of Materials Science and Engineering
> 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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Dr. Nektarios N. Lathiotakis
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Theoretical and Physical Chemistry Institute
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Athens, Greece
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