[Wien] different MLD for bcc structure for magnetic equivalent directions M001, M010 and M100

Mon Nov 27 15:27:08 CET 2017

Let me clarify slightly my comment about symmetry -- as I realized the
explanation (I think) and can also suggest something that might help.

First, concerning symmetry the explanation is I believe simple. If the
problem has a real symmetry operation such as inversion which is being
removed, then the Jacobian at the solution has zero's for charge
disturbances that break this symmetry. Because of this noise due to
numerical accuracy has a large effect, and almost certainly one has to
tighten the convergence criteria particularly -cc. You can monitor this by
looking at the :PUPXXX values in case.scfm and look how well the forbidden
reflections have converged to zero.

Second, do not be surprised about numerical issues. While the calculations
are done in double precision, there are many large sums and in some cases
double sums, and also numerical integrations/differentiation. Any large sum
or numerical integration/differentiation in general reduces the numerical
accuracy. Hence even though double precision has an accuracy of 1D-15 the
sum may only be accurate to 1D-10 or even 1D-7. Also, the Intel ifort
compiler will reduce the numerical accuracy for speed if one is not careful.

One thing that may help is to increase the oversampling in case.in0 for
VXC, both that of the PW's and of the CLMs. A standard test is to use LDA
and see if the problem goes away, since oversampling is much less relevant
for this.

Of course your problem may have nothing to do with any of this....

On Mon, Nov 27, 2017 at 8:07 AM, Jaroslav Hamrle <hamrle at karlov.mff.cuni.cz>
wrote:

> Dear all,
>
> thank you for your comments:
>
> 1)
>
> Did you use a Gamma centered k mesh (and enough k points)
>
>
> I have checked that the same inequality in MLD, appears both when k-points
> are shifted or not shifted. So, influence of shift of k-points can be ruled
> out.
>
> I have 30x30x30 k-points, which should be enough. When using 46x46x46
> k-mesh, the MLD inequality is reduced by about factor two, but still
> present. Even when using very fine k-mesh (90x90x90) for optical
> calculation, the inequality persists.
>
>
> 2)
>
> In some cells  shifting the k-point origin with MSR1a leads to slightly
> unbalanced forces which are hard to converge to the "right" symmetric
> result. If the forces are slightly off, this is an indication that the
> density is also slightly off. Exactly why this occurs I do not know, I
> suspect very soft modes associated with numerical errors in finite
> arithmetic.
>
>
> Well, for me it is hard to believe, that the problem can originates from
> numerical error. The MLD inequality creates sort-of ghost peaks in MLD
> spectra which for bcc Fe are very stable at positions at 4.8 and 6 eV for
> different calculation details (as different k-mesh, presence/absence of
> spin-orbit, shifted/non-shifted k-mesh). Under all those changes in the
> calculations, the position of ghost peaks remains very stable, just their
> amplitudes varies. Also, if problem would be just numerical one, why ghost
> peaks are not present in simple cubic or fcc calculations?
>
>
> 3)
>
>
> It is instructive to repeat the calculation without SO and see how big the
> difference between sig_xx and sig_yy (for any M direction) is then.
>
>
> I tried to calculate various combinations of structure, (with/without SO
> or sp) using full (non-magnetic) bcc symmetry, or bcc symmetry reduced by
> presence of magnetization (i.e. it means reduction of symmetry + new k-mesh
> as generated by initso).
> In case of spin-polarized calculations without SO, the ghost MLD peaks
> appear when going from full bcc symmetry to bcc symmetry reduced by
> magnetization.
>
>
>
> nosp+noso sp+noso sp+so
> bcc full symmetry without magnetization OK OK X
> bcc symmetry reduced by magnetization ? ghosts ghosts
> (fcc or simple cubic) reduced by magnetization
> ? OK OK
>
>
>
>
> Therefore it seems to me that the ghost MLD peaks appear when symmetry is
> reduced in the bcc structure. It seems that SO coupling is not important in
> this problem.
> It is the lower symmetry itself, which creates the MLD inequality (ghost
> MLD peaks).
>
> For example, can there be some small problem with generation of k-mesh or
> related symmetry in bcc+magnetization case?
>
>
> Thank you for your help
> With my best regards
> Jaroslav
>
>
>
>
>
>
>
>
> On 26/11/17 18:51, Laurence Marks wrote:
>
> I will third the comment that not using a shifted cell might be important
> (might). In some cells  shifting the k-point origin with MSR1a leads to
> slightly unbalanced forces which are hard to converge to the "right"
> symmetric result. If the forces are slightly off, this is an indication
> that the density is also slightly off. Exactly why this occurs I do not
> know, I suspect very soft modes associated with numerical errors in finite
> arithmetic.
>
> N.B., Wien2k is quite good with these numerical errors. I've noticed that
> Vasp calculations that collaborators have done often have much larger
> symmetry breaking.
>
> On Sun, Nov 26, 2017 at 11:39 AM, Karel Vyborny <vybornyk at fzu.cz> wrote:
>
>> I suppose that this does not have to do (much) with centering the mesh.
>> My guess based on other QMO calculations is that some contributions to
>> mat. els. of e.g. vx*vx from different parts of the BZ don't cancel
>> (numerically) even if they actually should.
>>
>> It is instructive to repeat the calculation without SO and see how big the
>> difference between sig_xx and sig_yy (for any M direction) is then.
>>
>> Cheers,
>>
>> Karel
>>
>>
>> --- x ---
>> dr. Karel Vyborny
>> Fyzikalni ustav AV CR, v.v.i.
>> Cukrovarnicka 10
>> Praha 6, CZ-16253
>> tel: +420220318459
>>
>>
>> On Sun, 26 Nov 2017, Fecher, Gerhard wrote:
>>
>> > There was a recent discussion on magnetic anisotropy, With a remark by
>> Peter,
>> > Did you use a Gamma centered k mesh (and enough k points)
>> >
>> > 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
>>
> --
> Professor Laurence Marks
> "Research is to see what everybody else has seen, and to think what nobody
> else has thought", Albert Szent-Gyorgi
> www.numis.northwestern.edu ; Corrosion in 4D:
> MURI4D.numis.northwestern.edu
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>
>
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>
> --
> ------------------------------------------------------------------
> Mgr. Jaroslav Hamrle, Ph.D.
> Institute of Physics, room F232
> Faculty of Mathematics and Physics
> Charles University
> Ke Karlovu 5
> 121 16 Prague
> Czech Republic
>
> tel: +420-95155 1340
> email: hamrle at karlov.mff.cuni.cz
> ------------------------------------------------------------------
>
>

-- 
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu ; Corrosion in 4D: MURI4D.numis.northwestern.edu
Partner of the CFW 100% program for gender equity, www.cfw.org/100-percent
Co-Editor, Acta Cryst A
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