[Wien] problems with convergence of SCF for AFM HoPtBi
Xavier Rocquefelte
xavier.rocquefelte at univ-rennes1.fr
Thu Nov 1 18:39:21 CET 2018
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.rocquefelte at univ-rennes1.fr
> <mailto: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
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