[Pw_forum] RE : RE : A question on atomic magnetic moments

[Giovani Faccin]

Dear Nicola,

Just read your article. Indeed, this dimer is not as easy as I first
thought!

I'm running a GGA+U study with U=3.1 eV (took that value from your article,
hope I understood it correctly). It's interesting that with GGA+U, integer
values are now appearing for the magnetization. For example, the first scf
interactions reported this:

interaction 1: 12.00
interaction 2:  4.00
interaction 3:  6.00
.
.
.
last interaction: 8.00

Although the value everyone gets is 6.00, the fact that integer numbers are
now showing up looks like progress. Smearing interference on the total
energy is not happening anymore (I'm using degauss = 0.0005 - thanks
Cyrille!)

Example 25 on the QE distribution talks about setting the
"starting_ns_eigenvalue" to explore different initial states on the energy
landscape. I'm wondering if, perhaps, that parameter could be the path to
reach a final 6.00 magnetization. I'll try to map different values of this
variable along with the system's total energy and see what shows up; if
something interesting is found, it'll be reported back here.

Unfortunately I've got no prior experience with some aspects of electronic
structure calculations. In particular, about the Kohn-Sham eigenstates
analysis you mentioned, I'm not sure what to look for there. If I'm not
being too much abusive, may I ask for some learning directions on this? (a
book or article suggestion, or perhaps an example?)

Thanks a lot!

Giovani Faccin



2011/2/15 Nicola Marzari <nicola.marzari at materials.ox.ac.uk>

>
>
> Dear Giovanni, Cyrille,
>
>
> the iron dimer has a very complex electronic structure (see e.g. our
> PRL 2006 Kulik et al) - different multiplets, and for each multiplet,
> different symmetries.
>
> In principle, there is no way to guess to which of the many
> self-consistent states you'll converge to, and I suppose that what
> you have here is that you have converged to different states.
> You'll need to look at the total energy, kohn-sham eigenstates, and the
> symmetry of those to understand what you are looking at
> (also, there is no point in having more bands than efermi + 3*degauss -
> empty bands above that threshold are useless).
>
> There is no easy way to force the code into one state or the other -
> you could try using GGA+U - some U's will stabilize reliably one state, and
> then by reducing smoothly the U (or increasing it) you might be able
> to have (as a function of U) the energy of your preferred electronic
> configuration.
>
> Last - for a cluster or a molecule smearing is used, somewhat
> inappropriately, to help a calculation converge when using
> iterative approaches - it keeps you company during the voyage,
> but shouldn't share your bed (the real reason d'etre for
> smearing is to improve k-point integrations in metals).
>
>                                nicola
>
>
>
> On 2/15/11 5:42 PM, Giovani Faccin wrote:
>
>> Dear Cyrille,
>>
>> I recalculated the dimer with different values of nbnd. Now some
>> interesting changes appeared:
>>
>> Value of nbnd    |   Magnetization
>>
>> 10     4.00
>> 12     6.88
>> 14     6.49
>> 16     6.49
>> 18     6.49
>> 20     6.49
>> 22     6.88
>> 24     6.88
>> 26     6.49
>> 28     6.49
>>
>>
>
> --
> ----------------------------------------------------------------------
> Prof Nicola Marzari    Department of Materials    University of Oxford
> Chair of Materials Modelling  Director, Materials Modelling Laboratory
> nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM
>



-- 
Giovani
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