[CPMD-list] orthorhombic space groups !!

[Łukasz Walewski]

Dear Imad,

I can give you a brief view based on my experience with isolated organic
molecules. They have some symmetry elements in the stationary points of
the energy surface. In the electronic ground state I did not use
symmetry at all, because optimizations converged very well. When I went
to excited states it turned out that the gradients in the vicinity of
the minimum are very small and the optimization was stopping before
reaching the real minimum (confirmed by vibrational analysis). I tried
to impose loose symmetry constraints to converge the geometry but the
symmetry module did not work for me at all. Optimization with CS
symmetry went to a geometry obviously lacking this symmetry element not
speaking of the higher symmetries like C2H which ended in floating
exceptions. The 'SYMMETRIZE COORDINATES' did not work for me either. I
wrote a simple program to symmetrize the coordinates myself and after
restarting optimization the geometry converged very well.

Axel, could you advise where to look for the symmetry routines in the
code ? Maybe I would have a look at that in my Free Time.

Regards,
Lukasz


Dnia 23-01-2008, śro o godzinie 20:22 +0100, Belabbas Imad pisze:
> 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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