Is dispersion included in _refln.F_calc?

[James Hester]

Dear Core DMG,

The addition of dREL to the DDLm dictionary means that we are forced to be
explicit about what we mean. Today's case is the application of dispersion:
do the F_calc values in a CIF file refer to F_calc with or without
dispersion?  I am told that modern software will always apply dispersion
where the numbers are available, so perhaps the answer is 'yes'.  With
historic CIF files, when working at a non-Xray-tube wavelength (eg at a
synchrotron), perhaps these values were not always computed and F_calc did
not include dispersion so the answer is 'no'.

As dREL methods aspire to compute a value for _refln.F_complex, it needs to
decide what to do based on the CIF file contents in the absence of the
explicit _reflns.apply_dispersion_to_Fcalc dataname. I suggest the
following logic

(1) If the wavelength is a recognised X-ray tube wavelength dispersion is
applied
(2) If the dispersion factors are supplied in the CIF file dispersion is
applied
(3) Otherwise, dispersion is not applied.

Please let me know your thoughts - does this correspond with how you and
your software worked both now and historically?

More background at https://github.com/COMCIFS/cif_core/issues/164

thanks,
James.
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