Hi Ian,
You're right the information is there... but not where I was expecting
it (on the page corresponding to an individual space group). It had
never occurred to me that it could be somewhere else.
So thanks, and regards to Jasmine.
Fred.
Ian Tickle wrote:
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
