[pdDMG] A definition for F_meas_squared for powder data
James Hester
jamesrhester at gmail.com
Mon Mar 13 04:43:43 GMT 2017
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_
--
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