[Wien] x symmetry problem

[Mr C.A. Bridges]

Sorry, there is one more issue I would like to clarify if possible. On
the WIEN2k faq page (http://www.wien2k.at/reg_user/faq/monoclinic.html),
the faq for monoclinic lattices says that the monoclinic angle must be
changed to near 90 degrees in order to run through symmetry.
 Would it be possible to suggest a good working value for all cases?
e.g. 90.01 or 90.1 deg?

Thank you,
Craig


On Fri, 21 Nov 2003, Mr C.A. Bridges wrote:

>
>
>On Fri, 21 Nov 2003, Peter Blaha wrote:
>
>>> I’m trying to run SCF cycle for a monoclinic TiO. In the data I have
>>> (program ICSD) for the space group is written A 1 1 2/m, #12. I saw that
>>> #12 in the StructGentm is B2/m. When I started xgroup I got that:
>>>
>>> sgroup found: 12 (C 2/m) [unique axis c] cell choice 2
>>>
>>> In the next step: x symmetry, I got:
>>> FORTRAN STOP -3 missing
>>
>>Centered monoclinic cells are only partially supported by WIEN.
>>The monoclinic angle MUST be gamma, so you must transform your spacegroup
>>representation into B2/m (basically interchanging some axis and positions)
>>and use this representation.
>>
>>Furthermore, please note that there is a yet unresolved bug for monoclinic
>>centered cells, which I can only summarize as: SOMETIMES the nonspherical
>>potential terms are wrong for this symmetry.
>>As SOMETIMES is not very specific, you MUST check your results using a
>>doubled P cell (or a triclinic P cell of same size, but no symmetry)
>
>
>Sorry, I would just like to clarify this for myself. When you refer to a
>'doubled P cell', do you mean that as the asymmetric unit of the
>primitive cell is twice that of the higher symmetry centered cell, you
>have to double the asymmetric unit size (rather than the entire cell)?
>And when you refer to a 'triclinic P cell of the same size', this is
>referring to a cell the same size as the monclinic centered cell?
>
>Basically, I am wondering if it is sufficient to lower the symmetry from
>C112/m (for example) to the subgroup P112/m? This would have the same
>size of unit cell (twice the size of asymmetric unit), and unique axis
>c.
>
>Please note for users of this list that there is a freely available
>windows program called "Powder Cell for MS-Windows" (clearly a windows
>program) on the site "http://www.ccp14.ac.uk" that will readily
>transform crystal structures between various space group settings.
>
>Craig
>
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