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 <[email protected]
<mailto:[email protected]>> 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>
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