[Wien] Increasing charge density for dense k-mesh.

[Peter Blaha]

 > *1. converged case.*
 >
 > In this case, I have taken a super cell of 2x2x1 made from a 225 space
 > group which contains 64 number of atoms. The composition is
 > Fe2VAl0.94Sn0.06 (Sn doped at Al-site of Fe2VAl alloy). For a 10
 > irreducible k-points (2x2x5), the calculations are converged by using a
 > mixxer value of 0.1 (case.inM)and TEMP 0.001 instead of TETRA 0.000 for
 > Brillouin zone sampling (case.2inc). But, after convergence, i want to
 > calculate the transport properties of system using Boltztrap code and
 > for that i need to have a dense k-mesh as suggested by the earlier
 > reports. Hence, i increased the k-mesh size/ irreducible number of
 > k-points to 41 (3x3x9). But, unfortunately, i have got an increase of
 > charge density approximately 2 orders greater for the first cycle of
 > run. Hence, i was waited to get the convergence and allowed to run the
 > calculation further. But, till now i have not get any charge convergence
 > and it is going to be diverge further as shown in below.

I'm not sure if I interpret your listings correctly, since it should not 
look like this.

After the first convergence (2x2x5) it seems the scf converged nicely.
(if these are the many lines with
 > :DIS  :  CHARGE DISTANCE       ( 0.0000711 for atom   12 spin 2)
 > 0.0000358

Then you should save it and continue with a better k-mesh. If your first 
:DIS is
 > :DIS  :  CHARGE DISTANCE       ( 0.0152057 for atom   44 spin 1)
 > 0.0085165

this seems perfectly ok. It tells you, that the new k-mesh will lead to 
a (slightly) different density and most likely you have to continue to 
final scf convergence. It is not unusual that a metal needs a finer 
k-mesh for a good convergence.

If your big :DIS
 > :DIS  :  CHARGE DISTANCE       ( 1.1613791 for atom   24 spin 1)
 > 0.9921918

happens AFTER the first convergence, it tells you that you need a MUCH 
finer k-mesh.



> ======================================================
>
> *2. Non-converged case.*
>
> In this case, i am trying to run the calculations for the composition
> Fe2VAl0.25Sn0.75 (Sn doped at Al-site of Fe2VAl alloy) where Sn
> concentration is more now. In this case, i have used primitive cell made
> from 225 space group that contains 16 number of atoms. We run the scf
> calculations after volume as well as force minimized structures with 32
> irreducible BZ k-points (4x4x4). To achieve the convergence, we tried
> several options as given below.
>
> 1. used mixxer values as 0.05 and 0.1, after saving previous calculation
> by save_lapw

If you are using a recent WIEN2k version, you should NOT play with the 
mixing values, at least NOT for your example.

Eventually increase TEMP 0.001 to 0.003 or 0.006   (magnetic ??)

When changing k-mesh or TEMP value, save the previous calculation.
(It removes broyden files)

Systems with transition metals (Fe) can be difficult to converge and not 
always a very small :DIS can be obtained. Check also :ENE or :FGL

In any case, if :DIS is so large (~1.0) starting from a converged calc., 
it means you need a much better k-mesh

> Note : Experimentally, i have also prepared these alloys, and got a
> segregation of pure Sn (extra phase) for higher compositions
> (Fe2VAl1-xSnx, x>0.2). Hence, formation of pure L21 phase is not
> achieved in the experimental case for higher compositions, while L21
> space group have been used for the calculation in the above case where x
> = 0.75.
>
> So, the divergence in the charge density for higher k-mesh size may
> conclude that it is not possible to get pure phase of L21 structure for
> this composition (Fe2VAl0.25Sn0.75). kindly, send me the comments on
> this observation.

No, I don't think that this would be a valid conclusion, also a it might 
be a "hint" (large DOS at EF)



-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
Email: blaha at theochem.tuwien.ac.at    WWW: 
http://info.tuwien.ac.at/theochem/
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