[pdDMG] A definition for F_meas_squared for powder data

[James Hester]

A further refinement to the definition based on feedback:

;
     The structure factor amplitude for the reflection derived by
partitioning
     the background-subtracted observed intensity _pd_proc.intensity_net
between
     reflections in the same proportion as those reflections contribute to
     the corresponding background-free calculated point in
_pd_calc.intensity_net .

;


On 21 March 2017 at 16:37, James Hester <jamesrhester at gmail.com> 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> 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 <+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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