[ddlm-group] How should we handle alternative models for F_calc?

(1) An obvious and easy to understand solution is to define new datanames, perhaps belonging to new categories, for each alternative model, so for a modulated structure we might have _refln_modulated.F_calc,  _refln_modulated.d_spacing etc.  An immediate consequence of this approach would be to require a new set of refinement statistics in refine_ls, as the current statistics are based on the match between refln.F_calc and refln.F_meas - so for each model, we would have a set of refinement statistic datanames. This would also be true for any other datanames that are derived from F_calc, e.g. difference density.  All of these datanames would be defined in the appropriate specialist dictionary.  Under such an approach, non msCIF-aware software would calculate F_calc and associated refinement statistics correctly, although these "average structure" values would be less useful for assessing data quality. 

(2) An approach that captures the current behaviour is to define a new dataname that labels the particular model used in a datablock (e.g. _audit.model).  As with _audit.schema, we advise all CIF input software authors to check the value of this dataname, and if it does not conform to the model that the software expects, then the software may exit or otherwise avoid silently performing an incorrect calculation.  This of course requires that authors of CIF output software include a value for this new dataname if they are using any model that is non-standard (i.e. not core CIF).  The two fundamental issues with this approach are (i) that it essentially redefines at least F_calc (although this is what is happening at the moment anyway) and (ii) that it contradicts a CIF principle that the value of one dataname should not change the way in which another is interpreted.  I believe that this principle is intended to encourage identifying separate concepts with separate datanames (i.e. approach (1)), although long-term COMCIFS observers may be able to shed more light on it.  In any case, COMCIFS, in its wisdom, may like/not like to consider overriding this principle for something as fundamental as the structural model.