:rename the file name to Two_Yukawa

And also update the document

sasmodels/models/TY_YukawaSq.py → sasmodels/models/Two_Yukawa.py

@@ -13,12 +13,14 @@
 
     \frac{V(r)}{k_BT} = \begin{cases}
     \infty & r < 2R \\
-    K_1 \frac{e^{-Z_1(r-2R)}}{r} + K_2 \frac{e^{-Z_2(r-2R)}}{r} & r \geq 2R
+    K_1 \frac{e^{-Z_1(r-1)}}{r} + K_2 \frac{e^{-Z_2(r-1)}}{r} & r \geq 2R
     \end{cases}
 
 where $R$ is radius_effective.
 $K_1$ ( or $K_2$ ) is positive when there is a repulsion.
 $K_1$ ( or $K_2$ ) is negative when there is an attraction.
+And $r$ is the normalized inter-particle distance, $\frac{r_{cc}}{2R}$. 
+$r_{cc}$ is the distance between the center-of-mass of particles.
 
 .. note::
 
@@ -45,24 +47,30 @@
 
 The algorithm used in this model was originally proposed and developled by Liu
 et al. in 2005 and implemented using MatLab.[1] The algorithm was later
-reimplemented using C and Igor. SasView uses the C codes developed by M. Hennig
+reimplemented using C and Igor. The codes for a two Yukawa potential in SasView are mostly based on the C codes written by M. Hennig
 in 2010.
 
 When the overall attraction is not very strong, this algorithm produces
 reasonably accurate results.[2] However, when the net attraction is very strong,
-it has been shown that the fitting algorithm tends to underestimate the
+it has been shown that the fitting algorithm tends to overestimate the
 attraction strength. Accuracy of this algorithm was evaluated by Broccio, et
-al.[2]
+al.[2] It should be noted that the MSA is not be an accurate closure for 
+systems with strong attractions. So cares need to be taken when having quantitative
+discussions of the fitting results for those systems.
 
 When using the O-Z equation to obtain the structure factor, it assumes that a
 system is in a liquid state. However, when the attraction potential is too
 strong the system may not be in a liquid state, and the O-Z equation may not
-have a solution, or the solution may be unphysical.
+have a solution, or the solution may be unphysical. It should be noted that any closure 
+for a O-Z euqation is an approximation. Thus, the structure factor calculated using a closure
+is an approximation of the real structure factor. Its accuracy varies for different parameter ranges.
+One should not "blindly" trust the results. This is true too for the MSA closure used here to solve 
+the O-Z equation for two Yukawa potential.[2]
 
 References
 ----------
 
-#. Y. Liu, W. Chen, S-H Chen, *J. Chem. Phys.*, 122 (2005) 044507
+#. Y. Liu, W. R. Chen, S-H Chen, *J. Chem. Phys.*, 122 (2005) 044507
 
 #. M. Broccio, D. Costa, Y. Liu, S-H Chen, *J. Chem. Phys.*, 124 (2006) 084501
 
@@ -79,7 +87,7 @@
 """
 from numpy import inf
 
-name = "TY_YukawaSq"
+name = "Two_Yukawa"
 title = "User model for two Yukawa structure factor (S(q))"
 description = """"""