# source:sasview/src/sas/sasgui/perspectives/fitting/media/fitting_sq.rst@332c10d

Last change on this file since 332c10d was 332c10d, checked in by smk78, 14 months ago

Create new help page for P@S models

• Property mode set to 100644
File size: 2.2 KB
RevLine
[332c10d]1.. fitting_sq.rst
2
3.. Much of the following text was scraped from product.py
4
5.. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
6
7.. _Product Models:
8
9Fitting Models with Structure Factors
10-------------------------------------
11
12.. note::
13
14   This help document is under development
15
16*Product models*, $P@S$ models for short, multiply the structure factor $S(q)$ by
17the form factor $P(q)$, modulated by the *effective radius* of the form factor.
18
19Many of the parameters in $P@S$ models take on specific meanings so that they
20can be handled correctly inside SasView:
21
22* *scale*:
23
24  The *scale* for $P@S$ models should usually be set to 1.0.
25
26* *volfraction*:
27
28  For hollow shapes, *volfraction* represents the volume fraction of
29  material but the $S(q)$ calculation needs the volume fraction *enclosed by*
30  *the shape.* SasView scales the user-specified volume fraction by the ratio
31  form:shell computed from the average form volume and average shell volume
32  returned from the $P(q)$ calculation (the original *volfraction* is divided
33  by *shell_volume* to compute the number density, and then $P@S$ is scaled
34  by that to get the absolute scaling on the final $I(q)$).
35
37
38  If part of the $S(q)$ calculation, the value of *radius_effective* may be
39  polydisperse. If it is calculated by $P(q)$, then it will be the weighted
40  average of the effective radii computed for the polydisperse shape
41  parameters.
42
43* *structure_factor_mode*:
44
45  If the $P@S$ model supports the $\beta(q)$ *correction* [1] then
46  *structure_factor_mode* will appear in the parameter table after the $S(q)$
47  parameters. This mode may be 0 for the local monodisperse approximation:
48
49    $I = (scale / volume)$ x $P$ x $S + background$
50
51    or 1 for the beta correction:
52
53    $I = (scale$ x $volfraction / volume)$ x $( <FF>$ + $<F>^2 (S-1) ) + background$
54
55    where $F$
56
57  More options may appear here in future as more complicated operations are