[Pw_forum] about C2/c point group symmetry of a monoclinic lattice.

Mutlu COLAKOGULLARI mutlucolakogullari at trakya.edu.tr
Sat Dec 20 22:40:21 CET 2014

Dear Andrea,
Thanks alot for your great explanation.
With my best wishes,
------------------------------------------Dr. Mutlu ÇOLAKOĞULLARITrakya Universitesi Fen FakultesiFizik Bolumu22030 Merkez-EDİRNE
> Dear Carlo, Andrea and Paolo;
>     The taking responds and advices from people who has experienced makes me happy.
>     As Paolo and Carlo also said, cif2qe.sh is not working properly, (at least for this case). Therefore, I used the other codes for double cross check: pymatgen and cif2cell. The conventional cell has 64 atoms whereas its primitive has 32 atoms. 
>     Pymatgen is very proper for POSCAR output because it takes symmetry analysis depend on Curtarolo-Setyawan's paper [http://arxiv.org/abs/1004.2974v1 (http://arxiv.org/abs/1004.2974v1)] (MCLC5 - it gives also gamma angle, for this case)...so, it is not matching with QE's ibrav settings depend on cell vectors. Cif2Cell code gives the exactly same CELL_PARAMETERS card with Quantum-ESPRESSO's ibrav=-13...that is the reason why I chose Cif2Cell code.
>     Andrea, it was my fault. I had been complicated among space groups(2/m for sg:12 and 2/C for sg:15) and Herman-Mauguin notations(2/m for space groups 12-15). You said that ?Using ibrav=-13 it is not necessary to add these atoms in the list of atoms inside the unit cell." Andrea, you mean that could I decrease the number of atoms in primitive cell if they have symmetrized by last 4 symmetry operations in this case? By this way, the cell lost half of atoms inside which is good news for computational resources. 
I think so. As far as I understand there are two possibilities:
1) You apply all the 8 symmetry operations and use ibrav=-12 (simple
monoclinic) obtaining 64 atoms.
2) You apply only the first four symmetry operations and use ibrav=-13
(one base centered monoclinic). Note however that in this case you have
to refer the positions to the base centered monoclinic axes, while the
positions that you obtain applying the symmetry are still referred to
the simple monoclinic cell. The base centered monoclinic cell has one
half the number of atoms of the simple monoclinic cell.

In no cases however QE will find 8 symmetry operations. In the first
case because fractional translations will be disabled (the cell  is
actually a supercell). In the second case because there are only 4.
It is not a question of notations, the point group for the space group
C2/c is C_2h (that has 4 operations), there is no way to obtain a larger
point group.


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