Hello Fred On 30 May 2012 07:55, Vellieux Frederic <frederic.velli...@ibs.fr> wrote: > Hi there,
> But: trigonal (and hexagonal) space groups "are" (usually?) polar. The cell > axis "c" can go "up" or can go "down", and in order to get a consistent > indexing you need to check both indexing systems when you scale additional > data to your native (the indexing chosen by your first crystals defines the > "standard" indexing - I must say that I haven't checked in the drawings of > the international tables if having c going up or going down leads to a > difference in that particular space group, P321, I'd need to draw both > possibilities and check but I'm sorry I do not have the time right now - in > fact it's too bad that the International Tables do not indicate "Polar" or > "Non-polar"). It does, at least my edition (Vol. A: 5th ed., 2002, Table 10.2.1.2, p.806) does - ITC has everything you need to know about space groups (and a lot more besides)! See also this table that I made where all polar & non-polar SGs are listed individually: http://www.ccp4.ac.uk/dist/html/alternate_origins.html Counting only the non-enantiomorphic ones, PG3 (4 SGs) and PG6 (6 SGs) are polar, whereas PG321 (3 SGs), PG312 (3 SGs), PG32 (1 SG) and PG622 (6 SGs) are non-polar. So in all 10 are polar and 13 are non-polar. A 2-fold axis perp to another axis always implies that there's no preferred direction along the other axis, so it's non-polar. Cheers -- Ian
