As already mentioned by jrh, I again want to impress that B-factor and occupancy are in principle two entirely different parameters.
They may appear as achieving the same thing upon refinement at the resolution we deal with in most macromolecular structures. The fact is that reduced occupancy just *scales down* the entire scattering function at a given B, while an increased B-factor *changes the shape* of the scattering curve, i.e. makes it narrower in reciprocal space and thus in direct space gives a broader electron density. Reduced occupancy just reduces electron density levels. Those are physically distinctly different although not easily separable effects. I have an applet that can be used to see the point http://www.ruppweb.org/cgi-bin/webscat.exe?E1=O <http://www.ruppweb.org/cgi-bin/webscat.exe?E1=O&B=50.0&Occupation_factor=0. 5&B1=Submit> &B=50.0&Occupation_factor=0.5&B1=Submit (disregard the core dump in the cgi) and a figure from the book also shows this: http://www.ruppweb.org/Garland/gallery/Ch9/pages/Biomolecular_Crystallograph y_Fig_9-06.htm Note that by reducing occupancy I can achieve a *similar* overall effect than by increasing B, but not the *same* electron density distribution! If the data are good enough (and reasonably high resolution), a refinement program can refine both parameters simultaneously, although with a high covariance. In summary, similar appearance is not caused by the same effect. Best, BR PS: Partial occupancy is not the same as disorder. You can have well-ordered different occupancies that manifest themselves then in superstructure patterns. Common in small molecule/materials. PPS: Neither n or B have anything to do with reflection spot width, btw! From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Pavel Afonine Sent: Monday, November 19, 2012 3:54 PM To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] occupancy vs. Bfactors Hi Grant, sounds like you did the right thing (as far as I can guess given the amount of information you provided). In a nutshell, both, B-factors and occupancies, model disorder. The difference is that occupancies model larger scale disorder (such as distinct conformations) than B-factors (smearing due to temperature vibrations, etc). Perhaps in you case the side chain has several conformations among which you can see only one, and therefore it's valid to model it with occupancy less than 1 (I presume you refined one occupancy per all atoms in that side chain). Pavel On Mon, Nov 19, 2012 at 3:36 PM, GRANT MILLS <gdmi...@students.latrobe.edu.au> wrote: Hello all, I'm currently working on a structure which if I stub a certain side chain phenix/coot shows me a large green blob which looks strikingly similar to the side chain, when I put it in and run another refinement the blob turns red. Basically I was just playing around and I changed the occupancy of the side chain and now there are no complaints. But I was thinking, should I haven changed the Bfactors instead? Should I have left well enough alone? If I lower the occupancy manually and do not include alternate confirmations have I introduced modelling bias? Could someone recommend some good articles I could read on exactly how to correctly fix this problem. Thanks, GM
