[Wien] Spin-polarization VS no spin-polarization in GaN

Fecher, Gerhard fecher at uni-mainz.de
Sat Apr 9 09:13:19 CEST 2016 

Your question "....  am I right to assume that for this material I don’t need to consider spin-polarization?"
causes several questions:

Why do you expect at all that you might need a spinpolarized calculation for GaN ?

What do you expect from a spinpolarized calculation for a material that has no magnetic order ?

What do you know about the difference between the two methods, spinpolarized or non-spinpolarized ?

Answer: There cannot be any difference when the spin resolved densities are identical for both spin channels !

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: wien-bounces at zeus.theochem.tuwien.ac.at [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von Marcelo Barbosa [marcelo.b.barbosa at gmail.com]
Gesendet: Freitag, 8. April 2016 23:03
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] Spin-polarization VS no spin-polarization in GaN

Thank you for both your emails.

Answering to the first one, yes, I forgot to write about the magnetic moments but they are zero in both calculations.

Answering to the second one, the forces were a little bit different (less than 1.0 mRy in difference) but I wasn’t expecting such a big difference in the EFG due to it, so I didn’t think that it was important to send them.
My mistake, I was wrong.
They were all in fact below 2 mRy, so the minimization stopped, but it stopped in different positions for each method.

I did a another minimization with both methods until the forces were all below 0.10 mRy (tried first with 1.0 mRy but it was not enough) and in this situation the results are much more similar:

:POS001 = 0.66667 0.33333 -0.00012 (no sp)
:POS001 = 0.66667 0.33333 -0.00022 (sp)

:POS002 = 0.66667 0.33333 0.37638 (no sp)
:POS002 = 0.66667 0.33333 0.37627 (sp)

:EFG001 = 0.41661 (no sp)
:EFG001 = 0.41404 (sp)

:EFG002 = 0.10461 (no sp)
:EFG002 = 0.10472 (sp)

I was doing these tests to see if I should use spin-polarization or not in calculations of GaN, specially for the calculation of the EFGs.
That’s why I was worried about the different EFGs that I was obtaining.
However, since they give the “same" results after all, am I right to assume that for this material I don’t need to consider spin-polarization?

Thank you both very much for your help,
Marcelo

On 08/04/2016, at 13:58, Peter Blaha <pblaha at theochem.tuwien.ac.at<mailto:pblaha at theochem.tuwien.ac.at>> wrote:

What you did not show was that the minimization will give different forces too.

The minimization will stop once the forces are for a couple of times below 2.0 mRy (case.inM) and the movement is small enough.

Even very small differences between the sp and non-sp calculation will lead to a different minimization-path and thus mixer will stop by chance at different positions leading to different EFGs (and E-tot).

In other words: If you would really want to get out the same results in both runs you need to put the cut-off in case.inM very small, so that mixer will optimize the positions until the forces are really small in all cases.


On 04/08/2016 12:41 PM, Marcelo Barbosa wrote:
Dear Sirs,

I did some calculations with GaN (struct file in attachment) to compare the difference in energy and electric field gradient between the use of spin-polarization or not.

The calculations were done using the commands

init_lapw -b -ecut -8 -numk 480 -rkmax 7 (-sp)
run(sp)_lapw -p -ec 0.00001 -cc 0.0001 -fc 0.5

and the results were the following

:ENE = -7995.60614872 (no spin-polarization)
:ENE = -7995.60614871 (spin-polarization)

:EFG001 = 0.57937 (no sp)
:EFG001 = 0.57931 (sp)

so they basically give the same.

Then, I relaxed the structure using

run(sp)_lapw -p -ec 0.00001 -cc 0.0001 -fc 0.5 -min

and the results were

:POS001 = 0.66667 0.33333 0.00002 (no sp)
:POS001 = 0.66667 0.33333 0.00000 (sp)

:POS002 = 0.66667 0.33333 0.37662 (no sp)
:POS002 = 0.66667 0.33333 0.37701 (sp)

:ENE = -7995.60615476 (no sp)
:ENE = -7995.60615035 (sp)

:EFG001 = 0.45038 (no sp)
:EFG001 = 0.58801 (sp)

so they gave different final positions and very different :EFG, as a consequence.

However, if I use the final structure from the minimization without spin-polarization and do a calculation with spin-polarization (both from the beginning, using the previous commands), I get equal values

:ENE = -7995.60615467 (no sp)
:ENE = -7995.60615466 (sp)

:EFG001 = 0.45036 (no sp)
:EFG001 = 0.45034 (sp)

and if I use the final structure from the minimization with spin-polarization and do a calculation without spin-polarization, I also get equal values

:ENE = -7995.60614842 (no sp)
:ENE = -7995.60614842 (sp)

:EFG001 = 0.58272 (no sp)
:EFG001 = 0.58271 (sp)


This means that the only thing that I am getting different between them is the atomic positions during the minimization, since I always get the same :ENE and :EFG if both calculations are done with the same struct file.

Is there any reason for this to happen?
Which values for :ENE, :EFG and :POS should I consider the best, the spin-polarized ones or the ones without spin-polarization?

Best regards,
Marcelo







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