[Pw_forum] how to find the space group and coordinates

[Eduardo Ariel Menendez P]

Hello,

I have a problem, that surely some of you have met before. It deals with
finding the space group and the crystal structure after an ionic
relaxation.

I have three lattice vectors and 40 atomic positions, and want to write this
structure acording to the International tables of crystallography. It
means that first, I need to find the space group. Second, the relaxed
atomic postions ARE NOT in the Wyckoff positions, and probably the  unit
cell is not the conventional cell. Hence, I need find the coventional
cell (can be a multiple or a submultiple), the origin of coordinates, and
the orientation of crystallographic axes, and the asymmetric unit. Then,
transform the coordenates to the new system of reference.

I have asked two crystallographers, and they want a diffracion pattern
to perform a structure refinement (why again?). I generated a powder
diffraction pattern, using a free
program (POWDERCELL), and put the diffraction pattern to a crystallography
refinement package that stated that the symmetry group of my system is
group 12 (C2/m). I think the conventional cell is double, i.e., 80 atoms.
Then, to find the atomic positions, it seems that I have to do a
refinement of the diffraction pattern. This task seems really difficult
and needs training. Furthermure, this route seems quite absurd to me.

Does anyone have a program that works directly with atomic coordinates?

Thanks

Eduardo A. Menendez Proupin
Department of Physics
Faculty of Science
University of Chile
Las Palmeras 3425
Ñuñoa, Santiago
Chile
Phone: 56+2+678 74 11
http://fisica.ciencias.uchile.cl/~emenendez/