[CPMD-list] orthorhombic space groups !!

[Axel Kohlmeyer]

imad,

i have only two comments (i am not a symmetry expert):

a) enforcing perfect symmetry is a rarely used feature
(most people use CPMD for MD where you want the opposite)
and thus always to be taken with a large grain of salt. please
see previous discussions here in the mailing list archive.
why are you so obsessed with it?
regardless whether CPMD detects the right symmetry, you
should get the right answer and i suspect the extra effort
due to not detected symmetry elements is less than the
time you have to spent to make them recognized.

b) please check your keyboard.
it looks as if the '!' and '?' key have a malfunction. ;-)

cheers,
    axel.

On 1/23/08, Belabbas Imad <belabbas_imad at yahoo.fr> wrote:
> 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 !!
>
>
>
>  ________________________________
>  Ne gardez plus qu'une seule adresse mail ! Copiez vos mails vers Yahoo!
> Mail
>
>
> _______________________________________________
> CPMD-list mailing list
> CPMD-list at cpmd.org
> http://cpmd.org/mailman/listinfo/cpmd-list
>
>
>


-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
  Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.