[Wien] core states in spin-polarized calculations and the open-core method

[Laurence Marks]

I think you have misunderstood the code. The lcore -up uses the "up"
potential, similarly for the "dn". That only 1/2 occupancies are used is
just book-keeping in order to not have a different code for spin-polarized
and non-polarized calculations.

As an aside, I don't trust +U  for 4f as it puts them inappropriately low
in energy. You can look at both DOI: 10.1103/PhysRevMaterials.2.025001 &
10.1021/acs.inorgchem.2c04107 . The experimental valence XPS data from many
authors is very clear that the energy of the 4f states is in the valence
regime, at least in oxides. Just my opinion.

--
Professor Laurence Marks (Laurie)
Department of Materials Science and Engineering, Northwestern University
www.numis.northwestern.edu
"Research is to see what everybody else has seen, and to think what nobody
else has thought" Albert Szent-Györgyi

On Mon, Jul 17, 2023, 06:51 Jindrich Kolorenc <kolorenc at fzu.cz> wrote:

> Dear Wien2k developers and users,
>
> I would be grateful for your opinion, clarification or pointers to
> literature about core states in Wien2k or LAPW in general. My
> message is a bit long, I am sorry for that.
>
> It seems to me that there is some inconsistency in the way Wien2k
> treats the core states in spin-polarized calculations. As far as I
> understand it, LCORE is run twice, once for spin-up potential and
> once for spin-down potential. In each of these runs, the core states
> are full (up and dn occupied). Then, I suppose, the up charge density
> is set to 1/2 of the LCORE charge run for the up potential and dn
> charge density is set to 1/2 of the LCORE charge run for the dn
> potential. That is different than running a core solver once, each spin
> channel feeling its own potential, which would seem to me as a more
> "rigorous" strategy (but one could not use the same quantum numbers
> for the core states as LCORE does).
>
> I suppose that different approaches to the core states have some effect
> also on the total-energy expression, since it contains sum of
> eigenvalues (I suppose since mixer reads them from case.scf). In
> Wien2k, there are two eigenvalues for each core state for
> spin-polarized calculations. I checked Elk, and if I understand its
> output correctly, it lists only one eigenvalue for each core state for
> spin-polarized calculations. That probably means, that Elk runs one
> core calculation for an averaged potential (though I did not
> investigate Elk in detail to know for sure).
>
> Do you know about any book or paper where different strategies would
> be discussed and/or tested/compared? My LAPW reference, the David
> Singh's book, does not appear to touch this.
>
> It may well be that for full core states all this is usually
> negligible, but I am more concerned about consequences for the
> open-core approximation for 4f states (which is where I actually
> noticed this "issue")
>
>   http://www.wien2k.at/reg_user/faq/open_core.html
>
> The FAQ entry does not mention spin-polarized calculations, but runsp
> does look for case.inc[up,dn] and it will use them if they exist. Then,
> fully spin-polarized polarized f^7 state is calculated as 1/2 (I
> suppose) of f^14 state, for instance. That is likely a good
> approximation for Eu^2+ charge density (full shell vs full subshell),
> but the eigenvalues/total energy are less obvious. And accuracy of
> approximating fully spin-polarized f^6 with 1/2 of spin-restricted f^12
> is even less obvious even for the charge density.
>
> I see that the FAQ entry says that one should use LDA+U instead of open
> core, but it has its own set of problems when applied to localized 4f
> too. And open core has one attractive feature for me - it gives an
> easy way to do calculations with constrained number of 4f electrons. Do
> you think it is meaningful to compare total energies for such
> calculations with different number of 4f electrons (charge-neutral
> cells, electrons shuffled between 4f and valence s,p,d).
>
> Any comments welcome.
>
> Best regards,
> Jindrich
>
>
>
>
>
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