-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Pavel, by artefact I mean density which is extremely difficult to interpret. When I worked in the K+ channels, especially the Tl version 1R3J, the difference density looks more pronounced than what I would have expected from ripples alone. As small molecules do not need to follow the symmetry, they tend to be disordered. If this is multiplied by four on a four fold axis, it becomes much harder than anywhere else in the asymmetric unit. I picked up from George Sheldrick that there are artefacts in the FT due to the proximity of special positions. As he usually knows what he is talking about, I trust his opinion, although there is a chance I misunderstood. Maybe you should discuss with him directly (I don't think he reads the phenixbb) for a clarification. Cheers, Tim On Friday, February 05, 2016 07:35:58 AM Pavel Afonine wrote:
Hi,
density on symmetry elements usually displays artefacts and is difficult to interpret.
I've seen this rumor perpetuated so many times.. I think I can also say "Fourier maps show artifacts anywhere in the unit cell volume (including symmetry elements) that originate, for example, from: a) finite resolution and partial completeness of the data, b) errors in crystal structure model (model is always used as a source of phase information), c) measurement errors. These artifacts are difficult (and unnecessarily) to interpret in terms of atomic model.". Are symmetry elements somewhat special in this regard (that is one sees more artifacts on symmetry elements than anywhere else)? If this is the case, can someone provide convincing reasoning for this (not hand-waving!)?
Thanks, Pavel
- -- Paul Scherrer Institut Tim Gruene - - persoenlich - OFLC/102 CH-5232 Villigen PSI phone: +41 (0)56 310 5754 GPG Key ID = A46BEE1A -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iD8DBQFWtfqSUxlJ7aRr7hoRAlU9AKCDDnCcInzLD1XxeZ3IfKQVyM8xZwCg626F qYwwnx0FVu4ZrV4frG3GIqE= =2IB2 -----END PGP SIGNATURE-----