.. fitting_sq.rst .. Much of the following text was scraped from product.py .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ .. _Product Models: 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 $($ + $^2 (S-1) ) + background$ where $F$ More options may appear here in future as more complicated operations are added. References ^^^^^^^^^^ .. [#] Kotlarchyk, M.; Chen, S.-H. *J. Chem. Phys.*, 1983, 79, 2461 .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ *Document History* | 2019-03-29 Paul Kienzle & Steve King