Hi Herman, On Thu, Oct 21, 2010 at 05:31:51PM +0200, herman.schreu...@sanofi-aventis.com wrote: > If you process your data in a lower symmetry space group, you will have > more unique reflections, since reflections which are related by the > higher symmetry will be avaraged during scaling in a higher symmetry > space group (i.e. a 2fold or 3fold axis), while in lower symmetry space > groups they will not. So the observation to parameter ratio stays the > same and is only depending on resolution and solvent content.
True - if you count Miller indices as observations. But if you think about information content than probably not (as you discuss below). > The question one has to ask of course is: are these reflections really > different, or are they the same only not averaged? Yes - by merging we're getting better data (better error estimate on the intensity due to higher multiplicity). So there isn't really independent information in 50% of the reflections if e.g. going from P21 to P1 - we've only increased the noise because the multiplicity of each reflection has been reduced. > In the latter case, you have more reflections, but not more > information. As Ed mentions, using tight "NCS" restraints would in > this case mimick the crystallographic symmetry. Apart from the (good) NCS argument, one could go even further: We could also just collect 36000 degree of data on a 7A Lysozyme crystal and refine against completely unmerged data. After all, why should we stop at removing only the some symmetry operators from our data merging ... lets get rid of all of them including th x,y,z operator and use unmerged data. Then we could refine Lysozyme with anisotropic hydrogens and no restraints against 7A data since we have a huge number of 'observations' ... right? But seriously: there is a difference in having reflections (H, K, L) and independent data (I, SIGI). Maybe we should talk more about (independent observations)/parameters ratio in the same way we look at depdencies of parameters (e.g. restraints on Bfactors etc). Cheers Clemens -- *************************************************************** * Clemens Vonrhein, Ph.D. vonrhein AT GlobalPhasing DOT com * * Global Phasing Ltd. * Sheraton House, Castle Park * Cambridge CB3 0AX, UK *-------------------------------------------------------------- * BUSTER Development Group (http://www.globalphasing.com) ***************************************************************
