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

[Nicola Marzari]

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.




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Prof Nicola Marzari   Department of Materials Science and Engineering
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