Although there is no need for a "standard" reciprocal asu, it is convenient to have all your datasets in the same convention when it comes to comparing and combining different isomorphous datasets (ie to do it once rather than every time you compare them). It doesn't matter what the "standard" is as long as it is consistent
Reading Ian's Email more carefully, it is true that eg in 321 the reindexing k,h,-l may take it out of the standard asu, and need an inversion operator to put it back. In that case, Pointless (and Reindex) do reduce the hkl to the asu and swap anomalous columns, and pointless will also invert phase columns etc >> No the hand-preserving transformation in 321 is (k,h,-l) > > But that's an equivalent of the space group so it won't transform from > the alternate setting (-h, -k, l). It will just give you a _different_ > a.u. of the _same_ setting. We need the _same_ a.u. of the > _different_ setting! Apologies, you are right, for 321 the reindexing operator is (-h,-k,l). But Pointless will do it correctly (I believe!) Phil On 29 May 2012, at 14:29, George Sheldrick wrote: > Which programs require that the data be the 'standard' a.u.? None of mine > require this. > > George > > On 05/29/2012 03:44 PM, Ian Tickle wrote: >> Mark, thanks for pointing that out, I see it now: >> >> In P321 the only possible alternate indexing is (-h, -k, l): this is a >> 2-fold || c which is an operator of the hexagonal lattice but is not >> an equivalent reflection. >> >> The standard CCP4 a.u. is h = k, l>= 0 or h> k, k>= 0, so for >> example (3,2,1) would be in the standard a.u. (3> 2 and 2>= 0). In >> the alternate indexing this would be (-3, -2, 1); however it's >> impossible to transform this to the a.u. with any non-inverting >> equivalent. The only possibility is to invert the hand, i.e. to (3, >> 2, -1) which is again in the a.u.. >> >> So the required re-indexing operator to match (3, 2, -1) with (3, 2, >> 1) is (h, k, -l) which reindex won't allow without the LEFT keyword >> (and you would be well-advised to avoid doing it with phase columns!). >> >> Cheers >> >> -- Ian > > -- > Prof. George M. Sheldrick FRS > Dept. Structural Chemistry, > University of Goettingen, > Tammannstr. 4, > D37077 Goettingen, Germany > Tel. +49-551-39-3021 or -3068 > Fax. +49-551-39-22582
