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Peter, I gave only the cases that have a single twin operator. The additional cases you refer to are all 'pseudo-merohedral' not 'merohedral' twins and require two or more twin operators. They are rare and in practice impossible to solve (and would require generating a special HKLF 5 reflection data file to refine with SHELXL). Since the pseudo-merohedral twinning is unlikely to be perfect, they would also tend to give split reflections. George Peter Zwart wrote: > *** For details on how to be removed from this list visit the *** > *** CCP4 home page http://www.ccp4.ac.uk *** > > >>In addition to the possibility that the true space group is P6 as >>suggested by Eleanor, it could also be P321 or P312, though the latter >>is uncommon. > > Actually, the full list of possibilities is a bit larger. > I have no clue how likely the obscure possibilities listed here are though. > > You find the complete (i hope so at least) list below. > possibilities are listed as > >>From X to Y using: > * Z > > where X would be a candidate 'true' point group Y the 'observed' point > group P622 and Z the twin law. > > Some point groups have funny names such as C 1 2 1 (2*y,-x+y,z). This > indicates that the point group is equal to C2, albeit in a somewhat > nonstandard setting [Note that C 1 2 1 (x+y,-x+y,z) is not equal to C 1 > 2 1 (2*y,-x+y,z)]. > > > > --------------------------------------------------------------------- > > >>From C 1 2 1 (2*y,-x+y,z) to P 6 2 2 using : > * h+k,-h,l > >>From P 6 to P 6 2 2 using : > * -h-k,k,-l > >>From C 2 2 2 (x+y,2*y,z) to P 6 2 2 using : > * -h-k,k,-l > >>From P 3 2 1 to P 6 2 2 using : > * -h,-k,l > >>From C 2 2 2 (2*x,x+y,z) to P 6 2 2 using : > * -h,h+k,-l > >>From C 1 2 1 (x+y,-x+y,z) to P 6 2 2 using : > * h+k,-k,-l > >>From C 1 2 1 (x+y,2*y,z) to P 6 2 2 using : > * -h-k,k,-l > > >>From C 1 2 1 (x-y,2*x,z) to P 6 2 2 using : > * h+k,-k,-l > >>From C 2 2 2 (x-y,x+y,z) to P 6 2 2 using : > * -h-k,k,-l > >>From P 3 1 2 to P 6 2 2 using : > * -h-k,k,-l > >>From C 1 2 1 (2*x,x+y,z) to P 6 2 2 using : > * -h,h+k,-l > >>From C 1 2 1 (x-y,x+y,z) to P 6 2 2 using : > * -h-k,k,-l > > > --------------------------------------------------------------------- > > For those who have interest, please find the full set point groups > between P1 and P622 in a graphical representation at > http://cci.lbl.gov/~phzwart/p622.png > > HTH > > Peter > > p.s. > > If you want to know how this image and list was generated, please > contact me personally > > > > > > > > -- Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-2582
