[Pw_forum] Small U for Co in ZnO for LDA+U
Stefano de Gironcoli
degironc at sissa.it
Mon Aug 4 13:00:12 CEST 2008
Dear all,
the occupation matrix for the LDA+U calculations is computed
starting from the atomic wavefuncitons which are ASSUMED to be
normalized...
Unfortunately a number of pseudopotentials (especially US ones) that
have been generated in the past did not enforce the normalization of the
atomic wavefunctions. This went mostly unnoticed because this properties
is not really needed in most situations were atomic wfc are used by
pw.x... but it is important in LDA+U and the of use these
pseudopotentials in LDA+U calculation results in "strange" occupations
which are basically wrong... (other projection options like
"ortho-atomic", or "norm-atomic", approximately orthogonalize the atomic
wave-fuctions but this is done in a way that 1) is crystal dependent 2)
makes technically more involved the calculation of forces and stress
that are therefore disabled).
To put an end to this problem in the CVS version (unfortunately not
in 4.0 nor in 4.0.1) normalization of atomic wavefunctions is tested and
enforced upon pseudopotential reading.
If you experience stange unexpected behaviors using LDA+U please make
sure to use the CVS version and report any problem that may still persist.
Hope this helps, best regards,
Stefano de Gironcoli
Nektarios Lathiotakis wrote
> 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
> <mailto: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
> <mailto:marzari at mit.edu> http://quasiamore.mit.edu
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>
> --
> --------------------------------------------------------------------
> Dr. Nektarios N. Lathiotakis
> --------------------------------------------------------------------
> Theoretical and Physical Chemistry Institute
> National Hellenic Research Foundation
> Vass. Constantinou 48, GR-11635,
> Athens, Greece
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> Inst. secretary: +30 210 7273792, +30 210 7273795
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