Dear all,

     It seems to me that the real question is not one of terminology
but of operational reality. 

     Operationally speaking, then, the fact that two crystals are
isomorphous means that one can meaningfully compare their structure
factors, or their amplitudes, in a reflection-wise manner, i.e.
F1(hkl) with F2(hkl), without having to involve any other reflections.
If this is at the limit of making sense but one somehow gets away with
it for certain purposes, then I guess one would talk about "poorly
isomorphous" crystals. If reflection-wise comparison breaks down, then
a change of lattice is required and one has a case of NCS, more
specifically of distinct crystal forms, requiring masking of the
contents of the ASU in one crystal form, rearrangement into an ASU for
the second one, and Fourier transformation. Writing this in the way it
was done in the early literature on NCS, this implements (up to fussy
details) a Shannon interpolation between the two transforms, which
reflects the fact that the structure factors or their amplitudes can
no longer be compared individually, hkl by hkl.

     It therefore seems to me that this operational distinction is
more important than the search for just the right terminology. Once
two crystals are not sufficiently isomorphous to allow a meaningful
reflection-wise comparison of structure factors, then they are
non-isomorphous, with "poorly isomorphous" in the twilight zone where
those paired comparisons are still possible but only to a resolution
significantly lower than that of the data.

     The reckoning of the degree of isomorphism by cell parameters
only should not be based of percentages of cell lengths, by the way:
it is the absolute differences of lengths, and their magnitude as a
fraction of the resolution limit, that determine whether pairwise
comparisons make sense or not.


     With best wishes,
     
          Gerard.

-
On Tue, May 31, 2016 at 10:59:20AM +0000, [log in to unmask] wrote:
> Being an optimist, I would use "almost isomorphous"
> 
> My 2 cts,
> Herman
> 
> Von: CCP4 bulletin board [mailto:[log in to unmask]] Im Auftrag von Bernhard Rupp (Hofkristallrat a.D.)
> Gesendet: Dienstag, 31. Mai 2016 11:11
> An: [log in to unmask]
> Betreff: [ccp4bb] Not isomorphous but similar is...?
> 
> Here is one for the definition buffs:
> 
> What do I call the relation between 2 structures of same SG that
> pack similarly but have too different cell constants/volume (5%)
> to be called isomorphous, but are still - what? Isostructural? Near-isomorphous?
> 
> Thx, BR
> 
-- 

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