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

[Jaroslav Hamrle]

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....