Effect of varying restraint strength on distribution of anisotropy

Categories of restraint:

Restraint toward isotropy
ISOR (shelxl) or SPHE (refmac)
tighter restraint forces the distribution to the right
Restraint toward similarity with adjacent atoms
SIMU (shelxl) or BFAC (refmac)
tighter restraint makes the distribution narrower and more symmetric
Restraint on along-bond component of ADPs
DELU (shelxl) or RBON (refmac)
kept constant at default value for this case study

A case study:

This figure shows the distribution of anisotropy among the 9697 protein atoms of a structure refined against 1.4Å data using SHELXL. Three refinement runs were made from the same starting point and differing only in the restraints applied to the ADPs of protein atoms.
Initial run
used the default restraint weights in shelxl (ISOR 0.1 SIMU 0.100)
the resulting distribution is skewed to the right, with a mean anisotropy of 0.524
No restraint toward isotropy
to shift the distribution to the left, the ISOR restraint was removed
to make the distribution more symmetric, the SIMU restraint was tightened slightly (SIMU 0.08)
the resulting distribution is skewed to the left, with a mean anisotropy of 0.393
Better choice of restraint weight
SIMU restraint is tightened further (SIMU 0.025)
the resulting distribution is now more symmetric, with a mean anisotropy of 0.446
This run has the lowest Rfree value, and the distribution is in good agreement with that of structures refined at much higher resolutions (shaded curve at bottom; see survey).