Concerning what Pierre asked, one point to remember at structure
refinement is to test and probably use TLS tensors. I had one case with
serious anisotropy in the data due a libration axis parallel to a
crystallographic axis. TLS lowered dramatically the R's and maps were neatly
better.
Concerning one point raised by Anastassis:
Think of a 1-2 micron thin plate, very well ordered, and an excellent
almost matching micro-beam:
shooting 'edge-on' you get a diffracting volume 'along' the plate and
data to - say - 2.5 A.
shooting 'face-on' you get a much lesser diffracting volume 'through' the
plate and data to - say - 3.2 A.
(if you shoot too much to get 2.5 you kill the crystal in no-time)
I remember one former good (small molecule ?) crystallography book with
words a kind of this "the crystals should be completely bathed by the x-ray
beam during the whole data collection" and also some other concerns about
beam homogeneity in its cross section. How serious is this nowadays ? Can
processing programs easily overcome, in a certain mounting, the fact that
not all crystal orientations have the same number of unit cells exposed to
x-rays ? What about inhomogeneities at the beam ? I understand that
technical difficulties may lead you to exposed your crystal partially to the
beam, etc..., but how hard should we care about this (how much effort to
avoid this) ?
Jorge
