[Wien] AFM type II

[Gavin Abo]

> * the definition for the compound a=b = 3.74 A alpha = beta = 90 gamma 
> = 120 *
> *for AFM type I : i creat  superstructure  x super cell target lattice 
> H  :  x =1,   y = 1 ,  Z =2  then x sgroup, program define 
> automatically the space group the same of my original space group 156 
> without warrning
> *

As you have described above (for Z=2), your attempt at creating a 
supercell has failed as "x sgroup" collapsed the supercell structure 
back to the non-supercell structure.

As mentioned on the FAQ page for supercell construction, you need to 
displace an atom, change an atom, or use a special label:

http://susi.theochem.tuwien.ac.at/reg_user/faq/supercells.html

In order to keep the supercell without "x sgroup" reducing it back to 
the original structure, refer to previous posts in the mailing list 
archive about breaking the symmetry.  A few of the many posts about that 
as examples are at the three links below:

https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg18380.html
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg01866.html
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg15517.html


> *but when try to do it for type II : x super cell  P :  x =1,   y = 1 
> ,  Z =1 x sgroup could not define the space group
> *

In section "3.12 Setting up a new case" on page 29 in the WIEN2k 19.1 
usersguide [ 
http://susi.theochem.tuwien.ac.at/reg_user/textbooks/usersguide.pdf ], 
there is the statement:

"Alternatively with the new StructGen you can specify the spacegroup and 
only the inequivalent positions. The equivalent ones will be generated 
automatically."

This means spacegroups in WIEN2k are defined according to the 
inequivalent positions and not by the equivalent positions.

For the case above (Z=1), it is likely that "x supercell" found some 
equivalent positions in the original structure and automatically added 
special labels to them changing them into inequivalent positions. Thus, 
a supercell structure was successfully created.  If you want the 
supercell structure to reduce back to the original structure, you would 
likely just need to remove all or some of the special labels in 
StructGen before running "x sgroup".

For understanding the inequivalent and equivalent positions with WIEN2k 
spacegroups, the example in the post at the following link might helpful:

http://zeus.theochem.tuwien.ac.at/pipermail/wien/2013-January/018171.html

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