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sasview/src/sas/sasgui/perspectives/fitting/media/fitting_sq.rst
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Fitting Models with Structure Factors
Note
This help document is under development
Product models, $P@S$ models for short, multiply the structure factor $S(q)$ by the form factor $P(q)$, modulated by the effective radius of the form factor.
Many of the parameters in $P@S$ models take on specific meanings so that they can be handled correctly inside SasView:
scale:
The scale for $P@S$ models should usually be set to 1.0.
volfraction:
For hollow shapes, volfraction represents the volume fraction of material but the $S(q)$ calculation needs the volume fraction enclosed by the shape. SasView scales the userspecified volume fraction by the ratio form:shell computed from the average form volume and average shell volume returned from the $P(q)$ calculation (the original volfraction is divided by shell_volume to compute the number density, and then $P@S$ is scaled by that to get the absolute scaling on the final $I(q)$).
radius_effective:
If part of the $S(q)$ calculation, the value of radius_effective may be polydisperse. If it is calculated by $P(q)$, then it will be the weighted average of the effective radii computed for the polydisperse shape parameters.
structure_factor_mode:
If the $P@S$ model supports the $beta(q)$ correction [1] then structure_factor_mode will appear in the parameter table after the $S(q)$ parameters. This mode may be 0 for the local monodisperse approximation:
$I = (scale / volume)$ x $P$ x $S + background$
or 1 for the beta correction:
$I = (scale$ x $volfraction / volume)$ x $( <FF>$ + $<F>^2 (S1) ) + background$
where $F$
More options may appear here in future as more complicated operations are added.
References
[1]  Kotlarchyk, M.; Chen, S.H. J. Chem. Phys., 1983, 79, 2461 
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