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Maybe I should mention the SHELX philosophy on restraints. The esds input with the restraints are presumed to reflect some sort of physical reality that is independent of the resolution of the X-ray data used in the refinement; they should not be changed for refinement with different datasets extending to different resolutions. SHELXL scales the weights attached to the X-ray data automatically relative to the restraint weights so that if the agreement of the X-ray data is poor (i.e. the R value is high) the X-ray data get a low weight, but this weight increases as the refinement progresses and the agreement with the X-ray data improves. There is no opportunity for the user to input a parameter to influence this. It is recommended that the restraints are retained even for very high resolution data, there will still be diffuse parts of the structure that need them. It is instructive to look at the estimates of the esds of for e.g. bond lengths that come out of the full-matrix inversion (L.S.): for parts of the structure (e.g. with low B values) that are well determined by the X-ray data, these esds are lower than the corresponding restraint esds, but for the less well defined regions the bond length esds asimptotically approach the restraint esds.
George -- Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-2582
