[CPMD-list] orthorhombic space groups !!

[Belabbas Imad]

Dear CPMD users and symmetry experts,

I am trying to treat a face centered orthorhombic molecular crystal (Beta-Acetonitrile). I have constructed a conventional unit cell by using the symmetry elements of the corresponding space group (Cmc21) taken from material studio. I have checked that the point group corresponding to my crystal is mm2 (C2v).  

I have made 2 tests:

* In the first test I have asked CPMD to determine automatically the point group.

* In the second test I have fixed the point group to C2v.


In the first case the code detects that there is 4 symmetry elements and the unit cell is not primitive (which looks fine !!). But it finds a point group: C2h !!!!! which, as far as I know is characteristic of a monoclinic system !!!. Moreover, in the supercell section it tells that the symmetry is Orthorhombic and the point group is C2h !!???. I have checked in the international tables of Crystallography, there is no Orthorhombic crystal that has a C2h point group !!!. Otherwise, in the KPTS_GENERATION file, CPMD uses the C2h point group !!

In the second case, where I forced the point group to be C2v, in the supercell section, the point group is mentioned as mm2 (C2v). However, in the KPTS_GENERATION file, CPMD usese the C2h point group !!

One more thing, CPMD recognizes an Orthorhombic unit cell if the cell parameters have the form [a b c 90 90 90] (absolute, degree). this constrains the user, in the case of  a face-centered  Orthorhombic, to use a conventional unit cell. Indeed, the  primitive unit cell has  lattice parameters like: [a a c alpha 90 90] (this looks like a monoclinic unit cell).
I am just wondering if this can explains why I am getting a monoclinic point group for my system !!!

Can someone comment on that !!???

Ps. I am attaching the input an output files of the two tests that I have performed + the CIF file of the crystal that I am treating !!   


             
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