[Wien] different MLD for bcc structure for magnetic equivalent directions M001, M010 and M100
Peter Blaha
pblaha at theochem.tuwien.ac.at
Fri Dec 1 11:09:07 CET 2017
I'm pretty sure the problem is connected with the tetrahedron method,
but also the presence of van Hove singularities and a k-mesh, which is
not converged.
a) As mentioned before, for all scf quantitites there is no problem if
one uses TEMP, but already the scf cycle differs when using TETRA.
b) At the end, this is not a big surprise. Checking case.kgen shows that
although the number of IBZ k-points is the same for both cases, the
number of tetrahedra (and their volume) is not. So clearly, the BZ of
these 2 cases are decomposed into a different set of tetrahedra, even
when you have the same number of k-points.
c) Naturally, from this it follows that all integrals over the BZ are
"different" (density, or the DOS and of course also joint), except when
the results are converged with respect to k-mesh.
d) I verified quite extensively that the matrix elements form optic obey
the expected symmetry.
e) I don't think that one can say that the BZ integration with
decomposition 001 or 100 is more or less accurate, therefore the only
thing which can help is to go to enourmous k-meshes until the quantity
you are looking for is converged (and then hopefully you obtain
identical results). After all, a 30x30x30 mesh is still a "lousy" mesh
for metallic optics (refer to Al).
f) I've done a 100x100x100 unshifted mesh. Also I've looked at the plot
without broadening. Clearly, your broad peaks at 5 and 6 eV are in fact
very sharp resonances, coming from some stupid van Hove singularities.
Such things are always very difficult to converge. In any case, my 1M
calculation has now the same sign (and also close magnitudes) for the
peaks at 5 and 6 eV (in contrast to the 30x30x30 calculation). In fact
all oscillations below 1 eV are now also correctly resolved and
"identical", only below 0.1 eV the magnitudes are still not identical
(but the sign is the same).
g) In order to do that, you probably don't need to do scf with these
meshes. Just do a scf cycle with eg. TEMP 0.001 and a good k-mesh.
The switch to TETRA 101.1 in case.in2c. Run kgen with better and better
meshes, and only lapw1/lapwso/lapw2/optic/joint in the proper sequence
h) Maybe ??? an alternative is to use a doubled P cell. But I'm not sure
if the results converge quicker with the k-mesh. Agreement between 001
and 100 directions does NOT necessarely mean that the results are
converged with respect to k-mesh !!
On 11/28/2017 12:36 PM, Jaroslav Hamrle wrote:
> Dear Laurence,
>
> thank you for your detailed answer.
>
> I have tried all your suggestions,
> - I changed case.in0 with increased oversampling by factor two (new
> parameters LUSE 26 and IFFTfactor 4)
> ------------ start of case.in0 -----------
> TOT XC_LDA (XC_PBE,XC_PBESOL,XC_WC,XC_MBJ,XC_REVTPSSS)
> NR2V IFFT 26 (R2V)
> 24 24 24 4.00 1 min IFFT-parameters, enhancement factor,
> iprint
> ------------ end of case.in0 -----------
>
> - I also tried to impose strong convergence criteria to be -cc
> 0.00000001 -ec 0.00000001
>
> However, in both cases, the ghost MLD remains practically identical as
> when using my default values (default Fe + convergence -cc 0.00001 -ec
> 0.000001)
>
> Also, final :PUP 'Current' parameters remained practically the same for
> all the calculations (by about 2 digits), being like (for M001)
> PW CHANGE H K L Current Change Residue
> :PUP001: 0 0 0 2.10719649E-02 5.090E-10 -7.530E-09
> :PUP002: 0 -1 -1 3.67084665E-04 -7.004E-10 -3.572E-10
> :PUP003: 1 -1 0 1.82124988E-04 -3.669E-10 -2.211E-10
> :PUP004: 0 0 -2 -1.87471938E-03 6.192E-11 7.254E-10
> :PUP005: 0 -2 0 -3.75090680E-03 1.125E-10 1.378E-09
> :PUP006: 1 -1 -2 -3.46372731E-03 1.026E-10 1.378E-09
> :PUP007: 1 -2 -1 -6.92804324E-03 2.418E-10 2.776E-09
> :PUP008: 0 -2 -2 -7.14014083E-04 8.677E-11 2.032E-10
> :PUP009: 2 -2 0 -3.57036516E-04 4.298E-11 1.121E-10
> :PUP010: 0 -1 -3 2.62159615E-04 9.199E-11 -1.588E-10
> :PUP011: 0 -3 -1 2.62289930E-04 9.844E-11 -1.555E-10
> :PUP012: 1 -3 0 2.62219244E-04 9.133E-11 -1.551E-10
> Unfortunately, all reflections seems to be allowed for bcc (H+K+L is
> even), forbidden reflections of bcc are (H+K+L=odd), so I can not see
> how they get close to zero ;-)) But the idea is excellent.
>
> Thank you again and with my best regards
>
> Jaroslav
>
> On 27/11/17 15:27, Laurence Marks wrote:
>> 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....
>
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--
P.Blaha
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