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

[Nicola Marzari]

Dear Nektarios,

two comments:

1) U is indeed pseudopotential dependent, especially if the
pseudos have been generated in different oxidation states. I.e.
there is not a unique U value - our tests in molecules show that
different pseudos give different U but the same chemistry - in
solids the situations is less clear cut, and not fully clear to me
at this stage. Note also that the U of pwscf is a U-J.

2) a U of 0.5 eV seems indeed small - i.e. does it have a significant 
effect with respect to U=0 ? If it has, there must be something very 
subtle going on.

3) tell the referee to get lost.


				nicola


Nektarios Lathiotakis wrote:
> Hi pwscf guys,
> 
> I have done some calculations on Co impurities in ZnO. I tried to fit 
> U_Co in such a
> way that the caluclated Co-d projected DOS to fit experiment
> (J. Appl. Phys. 103 07D130 (2008)). In this paper, they also fit U_Co.
> They use VASP. The problem is that the optimal U for pwscf and VASP differ
> quite a lot.
> 
> I found a projected DOS similar to the one in the above paper for
> U_Co = 0.5eV which is much smaller than their value (5.5eV with VASP).
> If I use their value, I get a Co-d projected DOS far deep into the valence
> (completely non sense).
> 
> Any idea why the optimal U values for Co in this case are so different?
> Unfortunately, their value is more common (most transition metals around 
> 5eV).
> Is it the different implementations for VASP and pwscf? Pseudopotential?
> 
> I have already tried to publish our results but the referees can not 
> accept the publication
> based on such a small U_Co value.
> 
> Interestingly, my fitted value is very similar to the one determined 
> self consistently by
> Hu et al, PRB 73, 245205 (2006) for the same system (0.35 eV). However, 
> a referee thinks that
> these small U numbers are "eroneous".
> 
> I include a typical input file with just 8 k points.
> 
> Thanks
> Nektarios

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Prof Nicola Marzari   Department of Materials Science and Engineering
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