Thank you so much Laurence for this explanation which definitely enlight
my understanding!
Could you just give me more hints. What do you mean by "fixed point
solution" when you are speaking about the spins?
Brainstorming session:
I am a chemist but I can propose an idea without a clear understanding!
The rotations are non-linear and we should find a way to express them in
such a way to be linear.
Is a Fourier transform (or fractional Fourier transform or something
related) useful in such a case?
Cheers
Xavier
Le 01/11/2018 à 17:19, Laurence Marks a écrit :
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.rocquefe...@univ-rennes1.fr
<mailto:xavier.rocquefe...@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
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Corrosion in 4D: MURI4D.numis.northwestern.edu
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