But that only means that the SF contribution from that pair of molecules in the unit cell is zero for (00l) l=odd reflections. Depending on the crystallographic symmetry (e.g. if it were 3-fold or higher order) there may be other pairs for which the SF contribution is non-zero for all (00l) (but zero for the the odd orders in some other line that is symmetry-equivalent to (00l)).
So for example I did a test with 2 mols in the a.u. related by an exact (0,0,1/2) NCS translation in P222: h k l Fcalc phicalc 0 0 1 0.00 180.00 0 0 2 5120.88 180.00 0 0 3 0.00 180.00 0 0 4 1273.09 0.00 0 0 5 0.01 0.00 0 0 6 260.21 180.00 0 0 7 0.00 180.00 0 0 8 39.42 180.00 0 0 9 0.01 0.00 0 0 10 43.19 0.00 0 0 11 0.00 180.00 0 0 12 118.59 180.00 0 0 13 0.01 0.00 0 0 14 120.89 180.00 0 0 15 0.01 0.00 (00l) l odd are definitely absent here! - because none of the crystallographic 2-folds affect the (0,0,1/2) translation. Then I generated 2 mols in the a.u. related by the (1/2,0,0) NCS translation in P4 to get the (h00) line (note that the 4-fold along c would not affect a c-axis translation): 1 0 0 25189.10 0.00 2 0 0 200.09 0.00 3 0 0 14404.46 180.00 4 0 0 12608.60 180.00 5 0 0 4669.64 180.00 6 0 0 748.81 0.00 7 0 0 3184.06 0.00 8 0 0 3858.87 0.00 9 0 0 2578.38 0.00 10 0 0 382.61 0.00 11 0 0 683.43 180.00 12 0 0 444.89 180.00 13 0 0 236.65 180.00 14 0 0 352.46 180.00 15 0 0 5.62 0.00 No absences! The 4-fold generates a pair of molecules related by the (0,1/2,0), not the (1/2,0,0) translation. So Eleanor is right only in specific cases, Ronald is right in the general case. As an aside, while I was verifying this, I think I found a problem with the P3 specific SF calculation in SFALL. Here's what I get for 2 mols in the a.u. related by an exact (0,0,1/2) NCS translation in P3 using the default P3 specific routine: 0 0 1 8.24 -175.63 0 0 2 7339.05 121.46 0 0 3 27.32 125.92 0 0 4 949.76 -10.37 0 0 5 39.10 -103.39 0 0 6 461.79 -115.99 0 0 7 19.18 61.53 0 0 8 219.72 -96.80 0 0 9 66.51 -174.45 0 0 10 288.25 -86.65 Now if I use the P1 routine (SFSG 1): 0 0 1 0.00 165.96 0 0 2 7343.28 121.51 0 0 3 0.00 -24.58 0 0 4 974.19 -13.35 0 0 5 0.00 -155.29 0 0 6 438.79 -115.60 0 0 7 0.01 133.49 0 0 8 226.07 -97.44 0 0 9 0.01 -101.82 0 0 10 291.15 -83.96 The 3-fold along c should not generate any pairs not related by the (0,0,1/2) NCS translation, so there should be (00l) l odd absences in this case. So something wrong here! Personally I always use the P1 routines, why take a chance just to shave a few secs off the run time? (note that ccp4i uses the default, i.e. the space-group specific routines). Cheers -- Ian > -----Original Message----- > From: owner-ccp...@jiscmail.ac.uk > [mailto:owner-ccp...@jiscmail.ac.uk] On Behalf Of Eleanor Dodson > Sent: 15 December 2008 09:56 > To: Ronald E Stenkamp > Cc: CCP4BB@jiscmail.ac.uk > Subject: Re: [ccp4bb] About system absence in P4222? > > Plug the coordinates (x,y,z) and x+tx, Y+ty, z+tz) into the > SF equation. > If the translation component along c is 0.5 say, then all the 00l > reflections for l=2n+1 will be absent. > > Eleanor > > > Ronald E Stenkamp wrote: > > 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 > >> > > > > > > > > Disclaimer This communication is confidential and may contain privileged information intended solely for the named addressee(s). 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