[pdDMG] A definition for F_meas_squared for powder data

[James Hester]

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> 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                          '_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                          '_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 <+61%202%209717%209907>
> F +61 (02) 9717 3145 <+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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