Re: [ccp4bb]: occcupancies

[Artem Evdokimov]

If they give you meaningful phasing, they're useful :) If they don't - they're not.   In other terms, if you can find the heavy atoms, then they're likely useful for phasing. If you can't find them easily enough then they probably do not contribute as much.   Having said that, a simple scenario where a few strong sites are found (classic derivatives of Pt, Hg, etc.) is quite different from a scenario where only weak (but numerous) sites are available (e.g. phasing on halogen ions). In the latter case, contributions of additional weak sites are supremely important for phasing.   In many programs the 'occupancy' parameter associated with the heavy atom appears to be an estimate which may or may not reflect on an actual real-life occupancy of the atom. Perhaps an interesting exercise would be to refine a structure from a heavy-atom dataset(s) and to estimate occupancies during refinement (at sufficiently high resolution SHELXL should do nicely) but there's never enough time for this kind of comparisons in my life :(   Artem   ----- Original Message ----- From: Vincenzo Carbone To: ccp4bb@dl.ac.uk Sent: Monday, October 31, 2005 7:08 PM Subject: [ccp4bb]: occcupancies As a general guide, at what levels of occupancy do heavy atom sites have to have before they become useful or too poor to use.