Hi Ron, If I understand your statement correctly NCS which does give rise to 'true' systematic absences must by definition be missed crystallographic symmetry. However, at the point of data reduction it may well be the case that the systematically weak reflections are hard to distinguish from those which are systematically absent. At the point of refinement there will clearly be a difference - one will give FC = 0, the other will give FC ~ very small.
I was recently looking at a data set which gave real issues with processing as it had pseudo-centering and the reflections which told you that this was not the case were so weak that autoindexing would usually not pick them up. That was fun. Cheers, Graeme -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Ronald E Stenkamp Sent: 11 December 2008 17:20 To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] About system absence in P4222? Hi. Non-crystallographic symmetry (NCS) doesn't apply to the entire crystal, so how can it give rise to systematic absences? I know it can give rise to systematically weak classes of reflections, but they aren't entirely absent. Ron On Thu, 11 Dec 2008, Winter, G (Graeme) wrote: > One of the many facilities in pointless is to search for absences and provide > a list of likely spacegroup choices based on the results. It includes > adjustments for neighbouring spots to address one of Eleanor's concerns. NCS > can cause reflections to be systematically absent too. > > The program can be found on the ccp4 prerelease pages or on the pointless ftp > site. > > Cheers, > > Graeme > > -----Original Message----- > From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of > Eleanor Dodson > Sent: 11 December 2008 09:41 > To: CCP4BB@JISCMAIL.AC.UK > Subject: Re: [ccp4bb] About system absence in P4222? > > 劉家欣(NTHU) wrote: >> Dear All: >> >> We have a crystl with P4222 sg. >> All statistics look fine. >> However, there is a system absense in l axis. >> Any body have experiences on that? >> Any suggestions would be high appreciated. >> >> jaishin >> > can you give more details, eg all reflections along the particular axis.. > Things like ice rings or overlapping intensity from a next neighbour getting > integrated inapropriately can cause anomalies.. > Eleanor >
