[QE-users] Incorrect identification+generations of atoms in special positions (space_group options used).

[Michal Husak]

I had carefully checked this mailing list and I had found several 
topic related to the same issue - QE generates multiple atoms for 
atoms in special positions.
Not solved in any answer.
What I mean:

General position:  x,y,z crystallographic position, atom not on a 
symmetry element (e.g. for spac egropu number 11, P21/m , 4 atoms are 
generated).

Special position: eg 2e x,z, (for space group 11, 2 atoms should be generated).
Atom is on the mirror element.

I had a structure described by CIF information in following way:

_symmetry_Int_Tables_number      11
_cell_length_a                   6.1103(2)
_cell_length_b                   20.6990(10)
_cell_length_c                   11.7877(5)
_cell_angle_alpha                90.00
_cell_angle_beta                 95.302(3)
_cell_angle_gamma                90.00

Zn 0.37830 0.2500 0.24657

The Zn is on the mirror

This can be transferred to QE input:

space_group = 11
uniqueb = .true.
A = 6.1103
B = 20.6990
C = 11.7877
cosAB = 0
cosAC = -0.092405345
cosBC = 0

And:
ATOMIC_POSITIONS crystal_sg
Zn 2e 0.37830 0.24657

Or
Zn 0.37830 0.2500 0.24657

Both input generate error = overlapped atoms.
What is wrong ?

Michal