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Dear Jacob, The resolution in reciprocal and real space was addressed by R. W. James in his paper 'False Detail in Three-Dimensional Fourier Representations of Crystal Structures' piblished in Acta Cryst. (1948). 1, 132-134. James showed that atomic density shape in the presence of series terminations is described by the Bessel function of zero order: 3*[sin(m) - m*cos(m)]/m^3 where m=2*Pi*r/dmin This function is equal to 1 at the atomic centre (r=0) and decreases as r increases, then it oscillates. The function reduces to 0.5 for r/dmin=0.4. Therefore, the two equal atoms to be separated (in a sense of having a density in between lower than at their centres) the distance between them should be equal (or higher) than 2*dmin*0.4 Main chain C and O atoms separated by 1.23 A should be resolved if dmin is better than 1.55 A. Two carbon sp3 atoms (distance 1.53 A) should be resolved if dmin is better than 1.9 A. Indeed, as Thomas wrote, 1.5 A is about right. The above is correct in the absence of atomic displacement parameters (Bfactor=0), for example for the normalised (sharpened) structure factor amplitudes. And, of course, for the complete and error-free diffraction data. Victor On 11/01/2018 21:07, Keller, Jacob wrote: > > The reason behind this query is that I want to illustrate the power of > prior knowledge in data analysis. I want to say something like “even > though atoms cannot be directly observed at worse than X resolution, > which represents Y% of the PDB, all of these data sets have been fit > correctly with atomic models. This is due entirely to the power of the > excellent priors which exist in crystallography.” > > JPK > > +++++++++++++++++++++++++++++++++++++++++++++++++ > > Jacob Pearson Keller > > Research Scientist / Looger Lab > > HHMI Janelia Research Campus > > 19700 Helix Dr, Ashburn, VA 20147 > > (571)209-4000 x3159 > > +++++++++++++++++++++++++++++++++++++++++++++++++ > > The content of this email is confidential and intended for the > recipient specified in message only. It is strictly forbidden to share > any part of this message with any third party, without a written > consent of the sender. If you received this message by mistake, please > reply to this message and follow with its deletion, so that we can > ensure such a mistake does not occur in the future. > > *From:* Thomas Edwards [mailto:[log in to unmask]] > *Sent:* Thursday, January 11, 2018 2:59 PM > *To:* Keller, Jacob <[log in to unmask]> > *Cc:* [log in to unmask] > *Subject:* Re: [ccp4bb] "Atomic resolution" > > Dear Jacob, > > Ah... this old chestnut! > > Current EM people say that they are at atomic resolution because they > are building atomic models (naive??). > > I have been criticised in the past for using the term with say 2.2A > diffraction data. By co-authors and reviewers alike. When I was young > and naive. > > My (current) definition would be yours - visible with data. > > I think 1.5A is about right for X-ray. Maybe higher res? > > I’m sure there are lots of rigorous ways to think. I probably haven’t > taken that route. However, I think it is a semantic problem that might > benefit from some disambiguation rather than rigour. > > It depends why you are asking the question... > > Sorry..! > > */Ed is: Out and about.../* > > */Sent from iPhone6sPlus./* > > > On 11 Jan 2018, at 19:31, Keller, Jacob <[log in to unmask] > <mailto:[log in to unmask]>> wrote: > > Dear Crystallographers, > > Has there been a consensus as to what is meant by “atomic > resolution?” Seems like the term is taken by various practitioners > to mean different things. > > A related question: at what resolution are atoms “visible” using > only the data? I have an empirical feeling that this would be > around 1.5 Ang Bragg spacings, but on the other hand, one can > contour up most maps and see individual atom peaks. I would be > interested to hear a more rigorous way to think about this. > > All the best, > > Jacob Keller > > +++++++++++++++++++++++++++++++++++++++++++++++++ > > Jacob Pearson Keller > > Research Scientist / Looger Lab > > HHMI Janelia Research Campus > > 19700 Helix Dr, Ashburn, VA 20147 > > (571)209-4000 x3159 > > +++++++++++++++++++++++++++++++++++++++++++++++++ > > The content of this email is confidential and intended for the > recipient specified in message only. It is strictly forbidden to > share any part of this message with any third party, without a > written consent of the sender. If you received this message by > mistake, please reply to this message and follow with its > deletion, so that we can ensure such a mistake does not occur in > the future. >