[Wien] different MLD for bcc structure for magnetic equivalent directions M001, M010 and M100
Peter Blaha
pblaha at theochem.tuwien.ac.at
Tue Nov 28 13:35:30 CET 2017
I looked into the two directories which you provided in a previous mail.
Why do you have 16 and 8 symm.ops in the corresponding struct files ???
Definitely you should also have in the 100 case 16 sym.ops.
I can therefore also see a vastly different number of k-points for the 2
scf files.
From you first email, I thought the problem is already in lapwso (or
the scf cycle), since you mentioned that already eigenvalues differ.
Personally, I found it already quite difficult to converge the 100 and
001 case to the same total energy using TETRA (there is no problem using
eg. TEMP 0.001). Depending on the k-mesh and the fermi-method I get
small oszillations (at low :DIS) on the mycroRy level and I would
probably have to go to larger meshes .....
This problem manifests itself in
grep :NEC01 case.scf (normalization not identical or oszillates ...) and
grep 'CHA ' case.scf
My two tests with 30x30x30 unshifted meshes and TETRA 101., RKMAX=7 gives:
:CHA : TOTAL VALENCE CHARGE INSIDE UNIT CELL = 8.072612
:CHA : TOTAL VALENCE CHARGE INSIDE UNIT CELL = 5.927386
13.999998
:CHA : TOTAL VALENCE CHARGE INSIDE UNIT CELL = 8.072635
:CHA : TOTAL VALENCE CHARGE INSIDE UNIT CELL = 5.927359
13.999994
Obviously it should always be exactly 14.0 (and this is fulfilled with
TEMP).
Using TEMP 0.0001 and tight convergence it is possible to get all
eigenvalues of a 100 and 001 calculations identical to 0.01 mycroRy (not
milliRy !!).
Of course, you have to be careful with this comparison, sind the same
k-points will NOT have the same eigenvalues, but symmetry-equivalent k
have: For instance for my test mesh, the 4th and 65th k-points are
equivalent, ...:
E(001)(k= 4 3 3 0 21) =
E(100)(k= 65 21 18 3 21)
Conclusion:
i) If k-space integration works, there is no problem whatsoever in the
scf cycle with/without SO, different symmetry, .....
ii) TETRA is dangerous in these comparisons, since the tetrahedra are
built differently for different reciplocal lattices and in particular if
there are subtle crossings at EF, the (non-)linear extrapolation may
lead to some artefacts. They are very small, but since SO for Fe is a
small effect, ...
------------------------------------------------------------------------
Later on, you focussed more on the optics (since this is your primary
interest) and claim, it has nothing to do with SO ???, only with
symmetry breaking. Maybe you are right, it is nothing than the
integration in joint using the weights from lapw2, which makes the
Problems. At least manual inspection of some momentum matrix elements,
which should be identical (or permutated) for equivalent k-pionts
indicates that they obey symmetry. ....
I checked your case.in2c files, and in both you have TETRA 0.000 as
Fermi method. As mentioned in the UG for optic one should NOT use the
Blöchl-method (with non-linear corrections), but use TETRA 100.1
and the original tetrahedron method.
Unfortunately, this does not fix the problems. Together with your
reports that with a larger k-mesh the problems get smaller, it points to
a problem in k-space integration in joint.
--------------------------
I'll continue to look into that problem.
On 11/27/2017 03:07 PM, Jaroslav Hamrle 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
>> <mailto: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
>> <http://www.numis.northwestern.edu> ; Corrosion in 4D:
>> MURI4D.numis.northwestern.edu <http://MURI4D.numis.northwestern.edu>
>> Partner of the CFW 100% program for gender
>> equity, www.cfw.org/100-percent <http://www.cfw.org/100-percent>
>> Co-Editor, Acta Cryst A
>>
>>
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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
> ------------------------------------------------------------------
>
>
>
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--
P.Blaha
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