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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 user-specified 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 $( <FF>$ + $<F>^2 (S-1) ) + background$

    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

Document History

2019-03-29 Paul Kienzle & Steve King
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