[Wien] Few questions about onsite hybrids and so

[Pavel Ondračka]

Dear prof. Marks,
thanks a lot for your comments, just some follow up (I was not sure
whether by "you can ask me offline" you meant private email, but
hopefully this can still be also interesting for the list):

On Mon, 2024-02-12 at 18:56 +0800, Laurence Marks wrote:
> Many comments/responses:
> a) You can do both forces and volume optimization with -eece, but not
> with -so.

Thanks for the clarification, this is very helpful, as I said I was
under the impression that I can relax O positions with so as long as I
don't turn on so for O ("UG 5.2.18 init_so_lapw: Since forces are not
correct for atoms with SO, it can be very useful to suppress SO for
light atoms (eg. the O-atoms in UO2 ), because then one can optimize
the O-positions.")

> b) For 4f what you did with case.in0eece is right, but check that it
> does not get overwritten. I had to edit an overwrite out of my
> runeece.

Will check, thanks.

> c) Expect the addition of -so to change things quite a lot -- and
> very little! The nett change in the energy will be very small, and
> you may want to think about the spin-ordering temperatures. Is your
> compound ferromagnetic, antiferromagnetic or what?

Honestly I have no idea, right now I have ferromagnetic, but this is
something I want to take a closer look at as well. It could be quite
complex, and maybe even questionable if I can even end with some
results that are relevant for the room temperature optical measurements
my colleagues are doing while within the limitations of collinear
model...

> d) People will tell you to use +U which will put the 4f electrons
> really low. My recommendation is to ignore them. As you noted they
> are in the valence regime.

Noted

> e) One way to fit the hybrid fraction is to get the best fit
> (approximately) to the x-ray positions. This turned out for me to be
> very reasonable.

Just to double check, by "X-ray positions" you mean refined atomic
positions from XRD or positions of the Tb states in XPS valence band
spectrum? XPS is something I definitely have on my TODO list.

> f) Beware too large RMTs. If you have these for the metal atoms then
> you get the tails of the O 2p states within those RMTs and that can
> give you artifacts.

To be honest I have no feeling here about what are too large RMTs in
this regard. I have 2.43 for Tb, 1.82 for Ga and 1.65 for O (this is
almost touching spheres). How big decrease would you recommend 5-10%?

> If you have other questions you can ask me offline if you want. You
> may want to look at DOI: 10.1103/PhysRevMaterials.2.025001,
> 10.1016/j.ultramic.2018.12.005, 10.1103/PhysRevMaterials.5.125002,
> 10.1021/acs.inorgchem.2c04107 Note that the XPS is dominated (cross-
> sections) by the 4f, and in TbScO3 that are at the Fermi edge (if it
> is Tb3+, Tb4+ will be simpler).

This is very exhaustive list, thanks. Will definitely read through it.

Best regards
Pavel

> 
> On Mon, Feb 12, 2024 at 6:15 PM Pavel Ondračka
> <pavel.ondracka at email.cz> wrote:
> > Dear Wien2k mailing list,
> > 
> > I'm trying to calculate a band structure of Tb3Ga5O12 magneto-
> > optical
> > crystal (cubic Ia-3d, 80 atoms). While I consider myself quite
> > experienced Wien2k user, I've always managed to stay away from f
> > block
> > elements, so my experience here is none. So besides the few
> > questions I
> > have I'll also try to somehow summarize what I did, please correct
> > me
> > if something was not OK.
> > 
> > Luckily I'm not shooting completely blind as I have some high-
> > quality
> > optical data where we can see some (very weak but also quite sharp
> > and
> > hence noticeable f-f transitions in the band gap so I have some
> > idea
> > how the Tb f states at least should look like). Significant optical
> > absorption start around 4eV but below that I see some very weak
> > electronic transitions in the 0.2-0.9eV range, around 2.5 and 3.5eV
> > (reportedly between f states located in the band gap). So I expect
> > at
> > least three bunches of f states in band gap one occupied and the
> > others
> > unoccupied.
> > 
> > I've started with spin-polarized PBE, I'm reasonably sure the
> > structure
> > file is OK, albeit probably not much relaxed (but I was hoping I
> > could
> > find equilibrium volume and do relaxation at a later point). I did
> > not
> > opt for HDLOs even though the Tb sphere is quite big (2.43) since I
> > would also like to try to get few momentum matrix elements later
> > with
> > optics, but I've increased the lmax to 14 and lvnsmax to 8 (lapw2
> > GMAX
> > 16, fft factor 3 and 4x4x4 k-grid).
> > 
> > The initial runsp went fine but the band structure is far from OK,
> > I
> > get only a single bunch of f states in the band gap clumped
> > together
> > (some of them are occupied so its metallic), but experimentally I
> > should get and insulator (although the difference between the
> > unoccupied and occupied f states in the band gap is only maybe
> > 0.2eV).
> > 
> > Regarding the f electron correction I opted for onsite hybrid and
> > initialized it with init_orb_lapw -eece.
> > UG says that its better to use LDA for the exchange potential so I
> > copied case.in0 to case.in0eece_lapw where I replaced "XC_PBE" on
> > the
> > first line with "EX_PBE VX_LDA EC_PBE VC_PBE".
> > The onsite hybrid calculation converged fine, I get a nice
> > splitting of
> > the f states (albeit a bit too much maybe).
> > The other options would be +U obviously, I went for the hybrid
> > because
> > it felt more rigorous, but I would also appreciate comments if
> > someone
> > has maybe better experience with +U?
> > 
> > Next step was to initialize spin-orbit interaction with
> > init_so_lapw. I
> > started with the default 001 but I want to also try other
> > directions
> > later and compare. I opted for no relativistic LOs (no support in
> > optics) and enabled it only for Tb and Ga. symetso created a new
> > structure (most notable I have more Tb inequivalent positions) and
> > than
> > I manually fixed case.inso case.indm and case.inorb as the init_so
> > script warned me. I also guessed I should fix case.ineece (that
> > seemed
> > straightforward) but than I thought I should also fix case.in2eece.
> > Reading UG gives the impression that case.in2eece is normal
> > case.in2
> > with extra EECE on the first line and than the optional 3a and 3b
> > lines. In the case.in2eece created automatically with init_orb_lapw
> > -
> > eece the 3a and 3b lines looked like:
> > 1
> > 1 1 3
> > However reading UG this actually seems wrong? Because UG says
> > (Section
> > 7.9 page 166) the format for optional 3b is just two values:
> > jatom rho, l rho
> > so I wonder if the UG is wrong or if I'm actually applying the
> > hybrid
> > correction to p instead of f?
> > 
> > Also, is there anything else I should fix manually after
> > intializing
> > the so on top of eece? Or should I do it the other way around
> > (first so
> > and then eece)? The reasoning for doing first eece was that I get a
> > metal with plain PBE and an insulator with the onsite hybrid, so I
> > thought it might be easier to converge if I start so from insulator
> > (but I still use TEMP smearing just to be sure I don't end with
> > convergence problems if I get a metal during the convergence as the
> > expected unoccupied occupied f-f distance is very small.)
> > 
> > I was also considering mBJ later, just to get some feeling how the
> > conduction bad would shift but I'm not sure if this would work or
> > not
> > on top of eece and so?
> > 
> > One last question is regarding the forces. From reading the UG I
> > understood that it should be OK to relax the oxygen positions with
> > onsite hybrid and so (as long as I don't have so or eece enabled
> > for O
> > atoms). Is this correct? So will just switching to MSR1a and
> > running
> > normal runsp -so -eece work or are some other fixes needed?
> > 
> > Best regards
> > Pavel
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> 
> 
> -- 
> Professor Laurence Marks (Laurie)
> Northwestern University
> Webpage and Google Scholar link
> "Research is to see what everybody else has seen, and to think what
> nobody else has thought", Albert Szent-Györgyi
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