WIEN2k-FAQ: Construction of supercells

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For most (simple) supercells you can start with the regular case.struct file and use the "supercell" program (x supercell) to generate the supercell-struct file.

For more complicated cases we also provide an octave-based program "structeditor", which is a very powerful and flexible tool to manipulate strcutures (for more info read the UG).

However, when you construct supercells, it is usually NOT possible just to double the cells and leave all atoms at their ideal and equivalent positions. This would lead to large multiplicities (e.g. in big supercells even more than 48 atoms could be "equivalent" and of course there is NO SPACEGROUP corresponding to this type of cell, since they all can be reduced by translation symmetry.

So during setup, you have at least to:

displace (at least one) atoms, or
change one of the atoms to a different type (usually you do supercells for impurities,... so put the impurity into the cell), or
set at least for one atom-name a special label (A label means an name like "Cr1", "Cr2" or "O 4", where in the 3rd position is a number. This indicates to WIEN2k that these atoms should be non-equivalent.).

Then run init_lapw

nn and sgroup determine which atoms are still equivalent (if nn does not agree with sgroup in the first place, you may have to "label" non-equivalent atoms (see above). If you now rerun nn, you may get another (better) list of non-equivalent positions.
symmetry: determines proper symmetry operations and rot.matrices,...
Check if your case.inst file is still valid (when the number of non-equivalent atoms has changed, probably not). Generate a proper case.inst file (either using "instgen_lapw" or simply by deleting case.inst) and rerun init_lapw.

Note, that WIEN always preserves symmetry. So when you want to allow certain relaxations around an impurity, it might be necessary to displace some atoms by hand to break the high symmetry and allow such relaxations.

Peter Blaha,