[Wien] Order of equivalent positions in case.struct

[pluto]

Hello Gerhard, Prof. Blaha,

Thank you for your comments.

I think equivalent atoms are always connected by symmetry operations.

Probably the easiest case is 2H WSe2 (or MoS2 etc.). Unit cell has two 
layers. One can sit at one of the K points, and then spin polarizations 
in subsequent layers are opposite. This is the "hidden spin 
polarization" of Zunger et al.

For bulk 2H WSe2 WIEN2k will make a case.struct file with 2 atoms (4 
equivalent positions for Se and 2 equivalent positions for W).

I believe QTL will take the first of the equivalent positions and 
provide properties inside the MT. In Wse2 one will get large spin 
polarization when looking at band characters with QTL.

Regular "x lapw2 -band -qtl" will average over all the equivalent 
positions, and one will get zero spin polarization. In order to get spin 
polarization with "x lapw2 -band -qtl", one needs to disconnect (split) 
atoms in case.struct. This will result in much bigger calculation. Not a 
problem for bulk WSe2, but very expensive for a big slab.

Of course, since equivalent atoms are connected by symmetries, one can 
find out these symmetries and then apply them to the properties such as 
spin or Ylm. But this may lead to some trivial mistakes, this is why I 
prefer to have the first equivalent atom on the same side of the slab 
(recently I do big slabs).

Best,
Lukasz




On 2024-09-02 08:34, Fecher, Gerhard wrote:
> Hallo Lukasz,
> If you edit the structure (or create a new one)  and use sgroup
> afterwards then it will resort the equivalent sites again when you
> accept the structure.
> If the space group was already ok, then you do not need to accept
> changes made by sgroup.
> I do not know whether there exists an IUCr recommendation how the
> equivalent positions should be ordered crystallographically. However,
> it should not affect the electronic structure.
> 
> I am not sure what you looking for, but how can two equivalent atoms
> contribute different to the electronic structure.
> For example, if the spin densities at two sites are different then
> they can not be equivalent.
> 
> Ciao
> Gerhard
> 
> DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
> "I think the problem, to be quite honest with you,
> is that you have never actually known what the question is."
> 
> ====================================
> Dr. Gerhard H. Fecher
> Institut of Physics
> Johannes Gutenberg - University
> 55099 Mainz
> ________________________________________
> Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von
> Peter Blaha [peter.blaha at tuwien.ac.at]
> Gesendet: Samstag, 31. August 2024 19:15
> An: wien at zeus.theochem.tuwien.ac.at
> Betreff: Re: [Wien] Order of equivalent positions in case.struct
> 
> Of course you can edit the struct file and exchange the order of
> equivalent positions.
> 
> Am 29.08.2024 um 13:41 schrieb pluto via Wien:
>> Dear Prof. Blaha, dear All,
>> 
>> WIEN2k automatically detects equivalent positions (from symmetries) to
>> allow faster calculations. In the slab geometry typically there are 
>> (at
>> least) two equivalent positions, at the top and at the bottom of the 
>> slab.
>> 
>> QTL is able to provide band characters for the first of the equivalent
>> positions. This is very convenient because one can check e.g.
>> spin-polarization of Rashba-type surface states without disconnecting
>> the atoms (which would be expensive). Regular x lapw2 -band will 
>> average
>> over the equivalent positions, therefore typically canceling any 
>> "hidden
>> spin polarization".
>> 
>> However, WIEN2k choses the first equivalent position automatically. 
>> This
>> means that sometimes it can be at the top of the slab, and sometimes 
>> at
>> the bottom of the slab (say surface layer is at the top and subsurface
>> layer at the bottom of the slab).
>> 
>> Is there a way to control which positions is the first equivalent
>> position? Can I manually edit case.struct file and change the order of
>> the equivalent positions?
>> 
>> Best,
>> Lukasz
>> 
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> --
> -----------------------------------------------------------------------
> Peter Blaha,  Inst. f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-158801165300
> Email: peter.blaha at tuwien.ac.at
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