[Wien] problems with convergence of SCF for AFM HoPtBi

[Laurence Marks]

Quite a few things are going on "Under the hood" in these systems, so let
me explain a little.

For heavy atoms with large RMTs the initial pseudocharge (PW density inside
the RMTs) from dstart is quite bad. In addition, unless you are careful to
chose a "good" initial spin state in case.inst, the spins can be quite far
from the fixed point solution. While it might appear that the density and
energy (:DIS and :ENE) are oscillating, in reality the pseudocharge and the
spin/orbital moments are rotating slowly in a N-Dimensional space.
Rotations are non-linear, and the mixer (all versions in all DFT codes to
my knowledge) use linear expansions. It can therefore take a long time for
these rotations tocomplete. What you will often see if you look at the
pseudocharge (grep :CPC in 18.1, the pseudocharge was not monited in
earlier versions as its role was not known) you will often see that it is
slowly "walking" -- similarly the spin moments etc.

When you add SOC and/or +U (or -eece) there will be large rotations of the
spins and/or spin moments.

In an ideal world some transformation would be applied to the Wien2k
variables to separate the pseudocharge and convert the rotations to linear
variables. I do not know how to do this, and simple ideas such as using the
sum and difference of spins don't work. (As expected, since this is a
linear transformation.) I am open to (mathematical)
suggestions/inspirations. It has to be math not fudge -- the mixer only
works when the math is right!

On Thu, Nov 1, 2018 at 9:34 AM Xavier Rocquefelte <
xavier.rocquefelte at univ-rennes1.fr> wrote:

> I have exactly the same experience. It is really a matter of cooking to
> my point of view and strongly depends on the system and what is
> happening at the Fermi level.
>
> In my case I am using DFT+U+SOC for such systems. The hubbard term
> indeed allows to separate the 4f states. However, the SOC is also
> essential and leads to large effects in these systems.
> In the case of HoPtBi, all the elements requires SOC. In addition it is
> an intermetallic compound, meaning that you have both localized and
> delocalized electrons!
>
> Did you try the following procedure:
> - DFT+SOC in a self-consistent manner
> - DFT+SOC+U on top of the first calculation
>
> Sometimes the lift of degeneracy due to SOC is sufficient to converge
> more efficiently DFT+SOC and then after DFT+SOC+U. And sometimes it is
> the other way around;)
>
> Best wishes
> Xavier
>
>
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-- 
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 ; Corrosion in 4D: MURI4D.numis.northwestern.edu
Partner of the CFW 100% program for gender equity, www.cfw.org/100-percent
Co-Editor, Acta Cryst A
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