On Tue, Jan 31, 2012 at 11:01 AM, Pavel Afonine
NCS: given state-of-the-art NCS restraints there is (probably) no clear-cut answer, but there are three ones: "definitely yes", "definitely no", and "try to find out". Obviously, at low enough resolution NCS should be always used (say ~2A and lower), simply because this provides a luxury of additional a priori information to alleviate the poor data-to-parameters ratio problem. Obviously, at high enough resolution (~1.5-1.7A or so) NCS should not be used since the amount of data may be enough to see actual differences between NCS copies, and using NCS would probably wipe out these difference (or at least there is such a risk). In the grey area, ~1.7-2.0A, one should try using vs not using NCS to know for sure.
A more quantitative answer: using the last six months' worth of PDB releases, for structures in the range 1.75 - 2.0 the best choice of NCS restraint (judged by R-free) was: none: 27% cartesian/global NCS: 11% (average improvement: 0.0073) torsion NCS: 62% (average improvement: 0.0036) However, some random checks (across a wider resolution range) indicate that a significant fraction of the structures where Cartesian NCS works best have under-assigned symmetry. Jeff is working on much more thorough tests, so a more definitive answer will eventually be available - but Pavel's recommendation will probably remain true. -Nat