There are many alternate origins for different space groups..
However a somewhat arbitrary choice or origin was made for the first
listing of independent space groups, and it is simplest to accept the
established conventions.
If you shift the C2 origin from 0 0 0 to 1/4, 0 0 you generate C21
Eleanor
wu donghui wrote:
> Dear all,
>
> I just get into touch with x-ray crystallography, and I made a "big"
> discovery that there is no C21 space group in monoclinic system, which
> system only contains P2, P21 and C2 groups. Even in the entire 65 chiral
> space group, there is no space for this group. I made an assumption that if
> there is C21 space group in monoclinic system, then the four equivalent
> point position could be described as: x, y, z; 1/2+x, 1/2+y, z; -x, 1/2+y,
> -z; 1/2-x, y, -z. However when I check the 65 chiral space group, no one is
> compatible with this equivalent positions. And I know I must make some
> mistakes. Unfortunately I can not find where the mistake stems from. I have
> to recourse to our expert community for your generous help.
> All replies are warmly welcomed and appreciated.
>
> Best regards,
>
> Donghui
>
>
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