Hello Holger,

I'm not sure to fully understand your question because I do not get what you mean by "if you enter the calculated position in a new calculation, you would obtain a wrong number of ions in the elementary cell" maybe you mean that your supercell is made of several unit cells and the fact that the atom sitting to the considered wyckoff position moves and enter in one of the unit cell you do not want.

To fix atom positions, you add in the POSCAR "Selective Dynamic" and at the end of each coordinates triplet relative to the atoms you write " T T T" to let the atom moves along the supercell based vectors or "F F F" to fix the positions.

Did you try to fix the atom you want to stay on the specific Wyckoff position with "F F F" and fix an additional atom of the system to prevent the drift of the over atoms ?

Matthieu

Hi Matthieu

thank you very much for your answer, which helped me a lot. I did not knew the "selective dynamics", which I tried know.

Just for clarification (maybe, I explained this too complicate): If you have in a crystal an atom at a general position, your symmetry operations produce from this generating atom a number of additional atoms (which is equal to the number of symmetry operations belonging to the space group).

If you have an atom at a special Wyckoff position, some space group symmetry operations transform this atom to itself and the number of atoms generated is smaller than in the general case.

If you make an ionic relaxation, it can happen, that the proposed position might be shifted from a special to a general position, which is hence contradictory to the crystal structure data.

Using "selective dynamics" gives a constraint to keep selected at special positions known from crystal structure determination.

Thank you again for your help

Holger

Hi Holger,

Ok,

Could it be that you generate a primitive cell of the system you are interested in, you relax it, and you are applying symetric operations to build a supercell ?

If so, is it for reduced the computational time when you will relax the supercell ?