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Re: [ccp4bb] Highest shell standards |
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CCP4bb navigationCCP4bb <-- 2007 <-- March 2007 <-- 23 March 2007Subject: Re: Highest shell standards From: Edward Berry eaberry {- at -} LBL {- dot -} GOV Date: 2007-03-23 > > I must admit that I've never understood the rationale for including dFc > for missing terms, although it has been discussed "lively" on a few > occassions. Yes, dFc is a better estimate for the true structure factor > than leaving out the term (equivalent to setting the amplitude to zero). > But dFc does not provide any information on how reliable the model is or > where the model may need to be changed. Since the latter is the main > function of electron density maps in my opinion I'm not confinced that > substituting dFc is a good idea at any time. > I believe fft does not by default do this- only if you use the "fillin" keyword? One place where it might be important is in density modification/ molecular averaging. Molecular averaging can be seen as a numerical solution of the MR equations, finding a density map which (A) obeys the NCS/intercrystal symmetry and (B) yields the observed F's upon Fourier transformation. Now if on each cycle you set the missing F's to zero, you are requiring it to have as part of (B) zero amplitude for the missing reflections, which is more restrictive and incorrect. If instead you allow the missing F's to "float", calculating them on each cycle from the previous map using the fillin option, someone has shown (don't have the reference handy at the moment) that the F's tend toward the true F's (in the case that they weren't really missing but omitted as part of the test). Ed CCP4bb navigationCCP4bb <-- 2007 <-- March 2007 <-- 23 March 2007 |
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