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