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Perpetual topic! The variety of opinions is amazing. Let me add my opinion here. I like the definition set by Z. Dauter, G. Murshudov, and K. Wilson in the International tables for Crystallography volume F (2001 edition), section 18.4.1 "Definition of atomic resolution". They define the atomic resolution as: when there are sufficient accurately measured observables to justify the refinement of the ordered part of the structure with full anisotropic ADPs. Of course, it has a different meaning than the optical resolution, but these tend to coincide. There is no magic number that fits all structures. It depends on the quality of data, completeness (especially in the highest shell), and solvent content. 1.2 A is a rather conservative estimate. To decide whether to use anisotropic ADPs or not, I like utilizing the Hamilton R-value ratio test. Basically, once you introduce extra parameters, the drop of the R-free should be significant in relation to the increase in the number of parameters. The concept is very well explained in: To B or not to B: a question of resolution? https://www.ncbi.nlm.nih.gov/pubmed/22505267 The test is implemented in HKL3000, and probably, in other software. If using the Hamilton test is not possible, i substitute the test with two independent considerations (both are present in Hamilton test): 1) (Number of Unique reflections)/(Number of non-hydrogen atoms)> 18 should put me on rather safe side. This ratio stands for "data-to-parameter ratio" = 2 or higher. One atom is defined by 9 parameters. 2) Drop of R-free has to be meaningful (significant). My rule of thumb is that at least 0.5% percent is good. 1% is safer. Ivan With best regards, Ivan Shabalin, Ph.D. Research Scientist, Department of Molecular Physiology and Biological Physics, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall,Room 4223, Charlottesville, VA 22908 On 01/11/2018 02:30 PM, Keller, Jacob 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. > ----