[Wien] Supercell problem of Al2O3

[Michael Fischer]

Hello again,

I hope I am able to explain this correctly without causing more confusion.
1.) For rhombohedral lattices in WIEN2k, you need to specify the hexagonal lattice parameters, but the rhombohedral atomic positions (User's Guide, 4.3). I think you had done this correctly in your original struct-file. This is why the angles are 90 degrees (gamma should be 120 deg, but I suspect that the angles are not even read because the program "knows" that gamma = 120 deg in a hexagonal lattice). Of course, rhombohedral lattices are defined as a=b=c and alpha=beta=gamma with the angles not equal 90 deg.
2.) If you want to use Cryscon, you need a consistent setting, i.e. both lattice parameters and atomic positions in rhombohedral setting. If you have no information about the rhombohedral lattice parameters, you have to re-arrange the formulae given in UG, 4.3, to calculate them from the hexagonal ones.
3.) Once you have done this, you can start Cryscon. Here, you create a new case and select Structure axes: Cartesian (choosing "trigonal rhombohedral" delivers strange results). Than you enter your cell parameters and the two atomic positions. Only two of them because all other atoms are generated by symmetry!. Choose the space group: 167:R. Ignore error messages.
4.) Now you can choose transform and change the transformation matrix to Rhomb->Hex. Click "Transform" again will give you an output. From this output, you get the lattice parameters (which you already knew) and the atomic positions. Although the program displays all atoms in the cell, you only need two of them: One Al and one O. Among others, you will find coordinates similar to those that I proposed in my previous posting:
Al (0 0 0.35)
O (0.30 0 1/4)
You can now enter these positions in structgen, and choose the correct space group. WIEN2k should do the rest.

Hope this helped you, greetings
Michael
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