[Wien] Questions about Na2IrO3 (PBE+SOC+U calculation with magnetic order)

[pieper]

A few thoughts on this:

ad 1)
Presumably Ir is the magnetic ion, so what you describe seems to be ok 
with me. I did not check your .struct file, however.

ad 2)
See chapters 4.5.6 and 7.4 of the UG (Wien2k 17.1) on ther orb-program, 
and the references therein. Personally I found the .pdf file of his talk 
on LDA+U very usefull that E. Assmann posted on the Wien2k-site.

Note the recommendation in the UG for the SIC-mode of orb for of 
strongly correlated systems: set J=0 and use only U_eff=U-J. The value 
of U_eff is something you will have to decide, perhaps based on the 
approach described by Madsen and Novak cited in the UG. You also might 
want to take a look at eece as an alternative (UG chapter 4.5.7)

 From many comments here in the mailing list and from the UG (again e.g. 
chapter 5.5.7, 4.5.8) I take it that PBE is what you should do if you 
want to calculate spacial charge and spin distributions, but to 
calculate gaps you may have to switch to numerically much more costly 
hybrid methods.

You probably can (mis)use U_eff as a free parameter to adjust the gap in 
your PBE calculation to your favorite value. However, the physical 
meaning of the value would be dubious (imho), and there is no guarantee 
that the Ir-moments simultaneously come near your favorit 'theoretical 
moments' (whatever the actual value and origin of those is).

ad 3) Don't bother with the starting values of local moments for atomic 
configurations, and for an antiferromagnet the interstitial moment 
obviously should stay close to zero.

But the moments you give in your table are very far from integer. Are 
this Ir-spin moments from  case.scf or did you add orbital moemnts 
calculated by lapwdm? The spin moments should be somewhere near integer 
for an insulator.

So, how did you determine the gap in the table? Did you plot a DOS? Is 
this really an insulator, or are there in fact bands crossing E_F? You 
might severly misjudge the (direct?) gap depending on where in k-space 
it is and the points in your k-list.

Good luck

---
Dr. Martin Pieper
Karl-Franzens University
Institute of Physics
Universitätsplatz 5
A-8010 Graz
Austria
Tel.: +43-(0)316-380-8564


Am 04.08.2017 19:22, schrieb Hung Yu Yang:
> Dear WIEN2k developers and users,
> 
> I am trying to do a calculation on Na2IrO3, which has a band gap
> ~340meV and a zigzag antiferromagnetic order in its ground state, and
> I have some questions as follow:
> 
> 1. To assign the zigzag antiferromagnetic order, what I did is that I
> first made a cif file that has two inequivalent Ir atoms, and let
> WIEN2k decide the symmetry for me. I adopted the generated structural
> file (attached in this mail), checked the cif file in some
> visulization software and made sure they were structurally equivalent
> except that there were 2 inequivalent Ir atoms instead of 1. Is this
> the proper way to generate structural files for magnetically ordered
> systems?
> 
> 2. After generating the desired structural file, I put up on Ir1 atom
> and down on the other (Ir2), used several different combinations of U
> and J, and I got the following results:
> 
> U(eV)         J(eV)         Ueff=U-J (eV)      Moment(Ir1, Ir2)
>   Gap(meV)
> 2.1             0.6             1.5
> 0.22532,-0.22439        ~700meV
> 2.4             0                2.4
> 0.30105,-0.30109        ~750meV
> 3                0.6             2.4
> 0.23225,-0.23235        ~900meV
> 3                1.5             1.5
> 0.17203,-0.17210        ~900meV
> 
> First, in this test, it seems that the gap is affected by U only, not
> Ueff=U-J. Second, the moments of Ir seemed to be closely related to J.
> In this situation, what is the proper way to assign U and J? I am not
> sure how much it means if I just try to tune U and J until they match
> the experimental gap and/or theoretical moments.
> 
> 3. I also have a question about the unit of moment given here; I
> understand that the moments depend on the RMT sizes and I assumed the
> unit is in bohr magneton, but the first value shown for Ir atom is
> 0.76697 and I am not sure in what unit is this value and how it
> assigns the initial moment for a certain element (say, Ir in my case).
> Also, the interstitial moments are nearly 0 among all the cycles.
> 
> For this calculation, kpoints=450, RKmax=8 and I had to use the TEMP
> scheme for it to converge. I appreciate any reply from you.
> 
> Yours sincerely,
> 
> Hung-Yu
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