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    before partitioning.
--
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Re: [pdDMG] A definition for F_meas_squared for powder data
- To: pddmg@iucr.org
- Subject: Re: [pdDMG] A definition for F_meas_squared for powder data
- From: James Hester <jamesrhester@gmail.com>
- Date: Tue, 21 Mar 2017 16:37:45 +1100
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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;
    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.
;
On 13 March 2017 at 15:43, James Hester <jamesrhester@gmail.com> wrote:
James.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,(New cif_pow definitions follow)save_refln.F_complex
_definition.id               Â         '_refln.F_complex'
loop_
 _alias.definition_id
        '_refln.F_complex'Â
        '_refln_F_complex'
_definition.update           Â         2017-03-07
_description.text            Â        Â
;
    The structure factor vector for the reflection calculated from
    the atom site data for the phase given by phase_id.
;
_name.category_id            Â         refln
_name.object_id              Â         F_complex
_type.purpose                Â         Measurand
_type.source                 Â         Derived
_type.container              Â         Single
_type.contents               Â         Complex
_enumeration.default         Â         0.
loop_
 _method.purpose
 _method.expression
        Definition        Â
;
        If (_diffrn_radiation.probe == "neutron") _units.code = "femtometres"
   Else If (_diffrn_radiation.probe == "electron") _units.code = "volts"
   Else                         Â                 _units.code = "electrons"
;Â Â Â Â Â Â Â
save_
save_refln.F_squared_meas
_definition.id               Â         '_refln.F_squared_meas'
loop_
 _alias.definition_id
        '_refln.F_meas'   Â
        '_refln_F_meas'
_definition.update           Â         2017-03-07
_description.text            Â        Â
;
    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            Â         refln
_name.object_id              Â         F_squared_meas
_type.purpose                Â         Measurand
_type.source                 Â         Derived
_type.container              Â         Single
_type.contents               Â         Real
_enumeration.default         Â         0.
loop_
 _method.purpose
 _method.expression
        Definition        Â
;
        If (_diffrn_radiation.probe == "neutron") _units.code = "femtometres"
   Else If (_diffrn_radiation.probe == "electron") _units.code = "volts"
   Else                         Â                 _units.code = "electrons"
;
save_
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