Changeset 4476951 in sasmodels

Timestamp:

Jun 28, 2018 9:09:47 AM (6 years ago)

Author:

Torin Cooper-Bennun <torin.cooper-bennun@…>

Branches:

master, core_shell_microgels, magnetic_model, ticket-1257-vesicle-product, ticket_1156, ticket_1265_superball, ticket_822_more_unit_tests

Children:

3b8a004

Parents:

5c36bf1

Message:

applied fix for ticket 1112 (allow negative rg)

File:

• sasmodels/models/guinier.py (modified) (3 diffs)

sasmodels/models/guinier.py

@@ -7 +7 @@
 .. math::
 
-    I(q) = \text{scale} \cdot \exp{\left[ \frac{-Q^2R_g^2}{3} \right]}
+    I(q) = \text{scale} \cdot \exp{\left[ \frac{-Q^2 R_g^2 }{3} \right]}
             + \text{background}
 
@@ -19 +19 @@
 
 .. math:: q = \sqrt{q_x^2 + q_y^2}
+
+Note that $R_g^2$ may be negative, which happens when a form factor $P(Q)$ is
+increasing with $Q$ rather than decreasing. This can occur for core or shell
+particles, hollow particles, or for composite particles with domains of
+different SLDs in a solvent with an SLD close to the average match point.
+(Alternatively, it might be regarded as there being an internal inter-domain
+"structure factor" within a single particle which gives rise to a peak in the
+scattering).
+
+To specify a negative value of $R_g^2$ in SasView, simply give $R_g$ a negative
+value ($R_g^2$ will be evaluated as $R_g |R_g|$).
+
+Note that the physical radius of gyration, of the exterior of the particle,
+will still be large and positive. It is only the apparent size from the small
+$Q$ data that will give a small or negative value of $R_g^2$.
 
 References
@@ -42 +57 @@
 
 #             ["name", "units", default, [lower, upper], "type","description"],
-parameters = [["rg", "Ang", 60.0, [0, inf], "", "Radius of Gyration"]]
+parameters = [["rg", "Ang", 60.0, [-inf, inf], "", "Radius of Gyration"]]
 
 Iq = """
-    double exponent = rg*rg*q*q/3.0;
+    double exponent = abs(rg)*rg*q*q/3.0;
     double value = exp(-exponent);
     return value;