Re: [pdDMG] A definition for F_meas_squared for powder data

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 <[email protected]> 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
F +61 (02) 9717 3145
M +61 (04) 0249 4148

--

T +61 (02) 9717 9907
F +61 (02) 9717 3145
M +61 (04) 0249 4148