Hello,
my question concerns refinement of a structure with 6-fold NCS (local
automatic restraints in REFMAC) against 2.8 A data. The size of my free set
is 1172 selected in thin resolution shells (SFTOOLS) and corresponding to
4.3 % of reflections.
A refmac run of 10 cycles of TLS and 10 cycles of CGMAT starts out at
Rfree/Rcryst 0.271/0.272. After the 10th TLS cycle I have 0.227/0.224. Yes,
Rfree < Rcryst. At the end of CGMAT I have 0.2072/0.2071.
I understand that NCS stresses the independence assumption of the free set.
Am I correct in believing that Rfree *may* be smaller than Rcryst even in
the absence of a major mistake? My hope is that the combined wisdom of
ccp4bb followers can point out my possible mistake, suggest tests that I
may perform to avoid them and, possibly, arguments in defense of a
crystallographic model with Rfree < Rcryst.
Many thanks,
Wolfram Tempel
