[pdDMG] A definition for F_meas_squared for powder data

[vondreele]

I'd only substitute 'profiles' for 'values' in the last sentence

Bob


On 3/21/2017 12:37 AM, James Hester wrote:
> Examination of the GSAS-II GetFobsSq routine suggests that a more 
> precise definition would note that the background should be subtracted 
> first.   I propose the following revised definition:
>
> ;
>      The structure factor amplitude for the reflection derived from a 
> powder
>      histogram by partitioning observed intensity between peaks at 
> each point
>      in the same proportion as the model peaks contribute to that point.
>      Model background is subtracted from both observed and calculated 
> values
>      before partitioning.
> ;
>
> On 13 March 2017 at 15:43, James Hester <jamesrhester at gmail.com 
> <mailto:jamesrhester at gmail.com>> wrote:
>
>     Dear Powder Dictionary Management Group,
>
>     It is now recognised formally that CIF dictionaries may need to
>     redefine datanames, and DDLm has been equipped with some
>     facilities to handle this.  In the case of the DDLm version of the
>     powder dictionary, as far as I can tell the only two datanames
>     from the core that are implicitly different are refln.F_complex
>     and refln.F_meas_squared.  _refln.F_complex is different purely
>     because the phase id has to be considered when performing the
>     calculation (unlike in the core case). _refln.F_meas_squared is
>     different both because of the need to consider phase_id, but also
>     because the procedure for calculating f_meas_squared from the
>     powder histogram is completely different to the single crystal
>     case.  Could you all please consider what I have written below for
>     F_meas_squared, and suggest any improvements. Ideally, the
>     definition text should be clear enough for a programmer to
>     implement the calculation.
>
>     While I would appreciate feedback from at least some of you, if I
>     don't hear from you within a week or so, I will assume that the
>     definition below is adequate.
>
>     all the best,
>     James.
>
>
>     (New cif_pow definitions follow)
>
>     save_refln.F_complex
>
>     _definition.id <http://definition.id> '_refln.F_complex'
>     loop_
>       _alias.definition_id
>              '_refln.F_complex'
>              '_refln_F_complex'
>     _definition.update 2017-03-07
>     _description.text
>     ;
>          The structure factor vector for the reflection calculated from
>          the atom site data for the phase given by phase_id.
>     ;
>     _name.category_id           refln
>     _name.object_id F_complex
>     _type.purpose Measurand
>     _type.source           Derived
>     _type.container           Single
>     _type.contents           Complex
>     _enumeration.default           0.
>     loop_
>       _method.purpose
>       _method.expression
>              Definition
>     ;
>              If (_diffrn_radiation.probe == "neutron") _units.code = 
>     "femtometres"
>         Else If (_diffrn_radiation.probe == "electron") _units.code = 
>     "volts"
>         Else _units.code =  "electrons"
>     ;
>
>     save_
>
>     save_refln.F_squared_meas
>
>     _definition.id <http://definition.id> '_refln.F_squared_meas'
>     loop_
>       _alias.definition_id
>              '_refln.F_meas'
>              '_refln_F_meas'
>     _definition.update 2017-03-07
>     _description.text
>     ;
>          The structure factor amplitude for the reflection derived
>     from a powder
>          histogram by partitioning observed intensity between peaks at
>     each point
>          in the same proportion as the model peaks contribute to that
>     point.
>     ;
>     _name.category_id           refln
>     _name.object_id F_squared_meas
>     _type.purpose Measurand
>     _type.source           Derived
>     _type.container           Single
>     _type.contents           Real
>     _enumeration.default           0.
>     loop_
>       _method.purpose
>       _method.expression
>              Definition
>     ;
>              If (_diffrn_radiation.probe == "neutron") _units.code = 
>     "femtometres"
>         Else If (_diffrn_radiation.probe == "electron") _units.code = 
>     "volts"
>         Else _units.code =  "electrons"
>     ;
>
>     save_
>
>     -- 
>     T +61 (02) 9717 9907 <tel:+61%202%209717%209907>
>     F +61 (02) 9717 3145 <tel:+61%202%209717%203145>
>     M +61 (04) 0249 4148
>
>
>
>
> -- 
> T +61 (02) 9717 9907
> F +61 (02) 9717 3145
> M +61 (04) 0249 4148
>
>
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