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Is dispersion included in _refln.F_calc?

  • To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group<coredmg@iucr.org>
  • Subject: Is dispersion included in _refln.F_calc?
  • From: James Hester <jamesrhester@gmail.com>
  • Date: Wed, 11 Mar 2020 12:09:32 +1100
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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?

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