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included S(Q) according to Henderson and Grundke

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Kohlbrecher committed Jul 5, 2018
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@@ -61,3 +61,4 @@ saskit/saskit_*
/src/plugins/sin2
/src/plugins/sin2/include/private.h
/src/plugins/sin2/include/sasfit_sin2.h
/src/fftw/fftw-3.3.8.tar.gz
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Analytical static structure factor for hard spheres} \hspace{1pt}
The rational function approximation method which is wholly compatible
with the equation of state used for the fluid has been used for expressions of the hard sphere structure factor.
In general, integral equation theories require a closure approximations, and they lead to different equations of state if one takes
the virial or the compressibility routes (thermodynamic inconsistency problem). In contrast the rational function approximation method
completely avoids the thermodynamic inconsistency problem, in that the compressibility factor is involved in the derivation of $S(Q)$ \cite{Yuste1996,Robles1997,Haro2004}.
\begin{align}
S(Q) &= 1-24\eta\Re\left(\left.\frac{t^2G(t)-1}{t^3}\right|_{t=\imath Q\sigma}\right) \label{eq:RFAstart}\\
G(t) &= \frac{t}{12\eta}\frac{1}{1-\exp(t)\Phi(t)} \\
\Phi(t) &= \frac{1+S_1 t + S_2 t^2 + S_3 t^3 + S_4 t^4}{1+L_1 t + L_2 t^2}
\end{align}
where the six coefficients $S_1$, $S_2$, $S_3$, $S_4$, $L_1$, and $L_2$ may be evaluated in an
algebraic form
\begin{align}
L_1 &= \frac{1}{2} \frac{η + 12\eta L_2 + 2 - 24\eta S_4}{2\eta + 1} \\
S_1 &= \frac{3}{2}\eta\frac{-1+4L_2-8S_4}{2\eta +1}\\
S_2 &= -\frac{1}{2} \frac{-\eta+8\eta L_2+1-2L_2-24\eta S_4}{2\eta +1} \\
S_3 &= \frac{1}{12} \frac{2\eta-\eta^2+12\eta^2L_2-12\eta L_2-1-72\eta^2 S_4}{\eta (2\eta+1)}
\end{align}
with
\begin{align}
L_2 &= -3\left(Z-1\right)S_4 \\
S_4 &= \frac{1-\eta}{36\eta\left(Z-\frac{1}{3}\right)}\left[1-\sqrt{1+\frac{Z-\frac{1}{3}}{Z-Z_\mathrm{PY}}\left(\frac{\chi}{\chi_\mathrm{PY}}-1\right)}\right]
\end{align}
Here, $Z_\mathrm{PY} = \frac{1+2\eta+3\eta^2}{(1−\eta)^2}$ and $\chi_\mathrm{PY} = \frac{(1−\eta)^4}{(1+2\eta)^2}$ are the compressibility factor and isothermal susceptibility arising in the PY theory. The isothermal compressibility $\chi$ and the compressibility factor $Z$ are related by
\begin{align}
\chi &= \left(\frac{\mathrm{d}(\rho Z)}{\mathrm{d}\rho}\right)^{-1} = \left(Z+\eta\frac{\mathrm{d} Z}{\mathrm{d}\eta}\right)^{-1}
\label{eq:RFAend}
\end{align}
where $\eta = \frac{\pi}{6}\rho \sigma^3$ is the packing fraction of the spheres ($\rho$ is the number density and $\sigma$ the
hard-sphere diameter).
The only quantity which is left to calculate the structure factor is an explicit expression for the compressibility factor $Z$.
\vspace{5mm}
\subsection{Carnahan and Starling} \cite{Carnahan1969}
\noindent In this approximation eqs.\ \ref{eq:RFAstart}-\ref{eq:RFAend} are used with $Z=Z^\mathrm{CS}$, where
\begin{align}
Z^\mathrm{CS} &= \frac{1+\eta+\eta^2-\eta^3}{\left(1-\eta\right)^3}
\end{align}
\vspace{5mm}
\hspace{1pt}\\
\underline{Input parameters for \texttt{Hard Sphere (CS)}:}
\begin{description}
\item[\texttt{R}] radius $R$
\item[\texttt{eta}] volume fraction $\eta$
\end{description}
\noindent
\underline{Note}
\begin{itemize}
\item The structure factor accepts volume fractions between $\eta \in [0,1]$.
\item The model lead to physically meaningful structural properties in the whole definition range of volume fractions.
\end{itemize}
\begin{figure}[htb]
\subfigure[Comparison between $S^\mathrm{PY}$ and $S^\mathrm{CS}$]{\includegraphics[width=0.46\textwidth]{../images/structure_factor/HardSphere/SQCS.png}}
\hfill
\subfigure[residual between $S^\mathrm{PY}$ and $S^\mathrm{CS}$]{\includegraphics[width=0.48\textwidth]{../images/structure_factor/HardSphere/ResCS.png}} \\
\caption{Comparison between analytical PY solution of hard sphere static structure factor and rational function approximation with a compressibility factor of Carnahan and Starling}
\label{fig:SQ:CS}
\end{figure}
\clearpage
\subsection{Pad\'{e}(4,3) of van Rensburg and S\'{a}nchez} \cite{Rensburg1993,Sanchez1994}
\noindent In this approximation eqs.\ \ref{eq:RFAstart}-\ref{eq:RFAend} are used with $Z=Z^{(4,3)}$, where
\begin{align}
Z^{(4,3)} &= \frac{1+1.024385\eta+1.104537\eta^2-0.4611472\eta^3-0.7430382\eta^4}{1-2.975615\eta+3.007000\eta^2-1.097758\eta^3}
\end{align}
\vspace{5mm}
\hspace{1pt}\\
\underline{Input parameters for \texttt{Hard Sphere (4,3)}:}
\begin{description}
\item[\texttt{R}] radius $R$
\item[\texttt{eta}] volume fraction $\eta$
\end{description}
\noindent
\underline{Note}
\begin{itemize}
\item The structure factor accepts volume fractions between $\eta \in [0,1]$.
\item The threshold packing fraction (packing
fraction at which a glass transition in the hard-sphere fluid takes place) of this model is $\eta^{(4,3)}_0 = 0.5604$ beyond
which no meaningful fluid structure can be derived \cite{Haro2004}.
\end{itemize}
\begin{figure}[htb]
\subfigure[Comparison between $S^\mathrm{PY}$ and $S^{(4,3)}$]{\includegraphics[width=0.46\textwidth]{../images/structure_factor/HardSphere/SQ(4,3).png}}
\hfill
\subfigure[residual between $S^\mathrm{PY}$ and $S^{(4,3)}$]{\includegraphics[width=0.48\textwidth]{../images/structure_factor/HardSphere/Res(4,3).png}} \\
\caption{Comparison between analytical PY solution of hard sphere static structure factor and rational function approximation with a compressibility factor of van Rensburg and S\'{a}nchez}
\label{fig:SQ:43}
\end{figure}
\clearpage
\subsection{Malijevsk\'{y} and Veverka} \cite{Malijevsky1999}
\noindent In this approximation eqs.\ \ref{eq:RFAstart}-\ref{eq:RFAend} are used with $Z=Z^\mathrm{MV}$, where
\begin{align}
Z^\mathrm{MV} &= \frac{1 + 1.0560\eta + 1.6539\eta^2 + 0.3262\eta^3}{\left(1- 3.8464\eta + 4.9574\eta^2 -2.1639\eta^3\right)\left(1-\eta\right)^3}
\end{align}
\vspace{5mm}
\hspace{1pt}\\
\underline{Input parameters for \texttt{Hard Sphere (MV)}:}
\begin{description}
\item[\texttt{R}] radius $R$
\item[\texttt{eta}] volume fraction $\eta$
\end{description}
\noindent
\underline{Note}
\begin{itemize}
\item The structure factor accepts volume fractions between $\eta \in [0,1]$.
\item The model lead to physically meaningful structural properties in the whole definition range of volume fractions.
\end{itemize}
\begin{figure}[htb]
\subfigure[Comparison between $S^\mathrm{PY}$ and $S^\mathrm{MV}$]{\includegraphics[width=0.46\textwidth]{../images/structure_factor/HardSphere/SQMV.png}}
\hfill
\subfigure[residual between $S^\mathrm{PY}$ and $S^\mathrm{MV}$]{\includegraphics[width=0.48\textwidth]{../images/structure_factor/HardSphere/ResMV.png}} \\
\caption{Comparison between analytical PY solution of hard sphere static structure factor and rational function approximation with a compressibility factor of Malijevsk\'{y} and Veverka}
\label{fig:SQ:MV}
\end{figure}
\clearpage
\subsection{L\'{o}pez de Haro and Robles} \cite{Robles2003}
\noindent In this approximation eqs.\ \ref{eq:RFAstart}-\ref{eq:RFAend} are used with $Z=Z^\mathrm{LHR}$, where
\begin{align}
Z^\mathrm{LHR} &= \frac{1 + 0.153555\eta
- 0.428376\eta^2
- 2.7987\eta^3
- 0.317417\eta^4
- 0.105806\eta^5}{1-3.84644\eta + 4.9574\eta^2 - 2.16386\eta^3}
\end{align}
\vspace{5mm}
\hspace{1pt}\\
\underline{Input parameters for \texttt{Hard Sphere (LHR)}:}
\begin{description}
\item[\texttt{R}] radius $R$
\item[\texttt{eta}] volume fraction $\eta$
\end{description}
\noindent
\underline{Note}
\begin{itemize}
\item The structure factor accepts volume fractions between $\eta \in [0,1]$.
\item The threshold packing fraction (packing
fraction at which a glass transition in the hard-sphere fluid takes place) of this model is $\eta^\mathrm{LHR}_0 = 0.5684$ beyond
which no meaningful fluid structure can be derived \cite{Haro2004}.
\end{itemize}
\begin{figure}[htb]
\subfigure[Comparison between $S^\mathrm{PY}$ and $S^\mathrm{LHR}$]{\includegraphics[width=0.46\textwidth]{../images/structure_factor/HardSphere/SQLHR.png}}
\hfill
\subfigure[residual between $S^\mathrm{PY}$ and $S^\mathrm{LHR}$]{\includegraphics[width=0.48\textwidth]{../images/structure_factor/HardSphere/ResLHR.png}} \\
\caption{Comparison between analytical PY solution of hard sphere static structure factor and rational function approximation with a compressibility factor of L\'{o}pez de Haro and Robles}
\label{fig:SQ:LHR}
\end{figure}
\clearpage
\subsection{Grundke and Henderson} ~\\
\noindent In this approximation the structure factor in the Percus Yevick approximation is corrected to have thermodynamic consistency \cite{Henderson1975} so that both routes leads to the expression of Carnahan and Starling
$pV/Nk_BT=\frac{1+\eta+\eta^2-\eta^3}{\left(1-\eta\right)^3}$
\begin{align}
\sigma &= 2R\\
\left(\sigma_0/\sigma\right)^3 &= 1-\eta/16\\
\eta_0 &= \eta (1-\eta/16) \\
g_0(s,\eta_0) &\simeq \frac{1+\eta_0/2}{\left(1-\eta_0\right)^2} - \frac{9}{2}\eta_0\frac{1+\eta_0}{\left(1-\eta_0\right)^3}(s-1) \\
\frac{C}{\sigma} &= \frac{2-\eta}{2(1-\eta)^3} - g_0(\sigma/\sigma_0,\eta_0) \\
\frac{12\eta C}{m\sigma_0^2} &= \frac{(1-\eta)^4}{1+4\eta+4\eta^2-4\eta^3+\eta^4} - \frac{(1-\eta_0)^4}{(1+2\eta_0)^2} \nonumber \\
&= 24\eta_0\int_0^{\sigma/\sigma_0} g_0(s,\eta_0)s^2 \mathrm{d}s \\
S^\mathrm{GH}(Q,\sigma,\eta) &= S^\mathrm{PY}(Q,\sigma_0,\eta_0) + \frac{6\eta}{\pi\sigma^3}\tilde{h}_\mathrm{GH}(Q) \\
\tilde{h}_\mathrm{GH}(Q) &= -\frac{4\pi\sigma_0}{Q\sigma_0} \int_1^{\sigma/\sigma_0} sg_0(s,\eta_0)\sin(Q\sigma_0 s)\mathrm{d}s\\
&+ \frac{2\pi\sigma^3}{Q\sigma}\frac{C}{\sigma} \left\{ \frac{\cos (Q\sigma)}{\sigma}\left[\frac{k+m}{m^2+(Q+m)^2}+\frac{Q-m}{m^2+(Q-m)^2}\right]\right.\nonumber \\
& + \left. \frac{\sin (Q\sigma)}{\sigma}\left[\frac{m}{m^2+(Q+m)^2}+\frac{m}{m^2+(Q-m)^2}\right]\right\}\nonumber
\end{align}
\vspace{5mm}
\hspace{1pt}\\
\underline{Input parameters for \texttt{Hard Sphere (GH)}:}
\begin{description}
\item[\texttt{R}] radius $R$
\item[\texttt{eta}] volume fraction $\eta$
\end{description}
\noindent
\underline{Note}
\begin{itemize}
\item The structure factor accepts volume fractions between $\eta \in [0,1]$.
\end{itemize}
\begin{figure}[htb]
\subfigure[comparison between $S^\mathrm{PY}$ and $S^\mathrm{GH}$]{\includegraphics[width=0.46\textwidth]{../images/structure_factor/HardSphere/SQGH.png}}
\hfill
\subfigure[residual between $S^\mathrm{PY}$ and $S^\mathrm{GH}$]{\includegraphics[width=0.48\textwidth]{../images/structure_factor/HardSphere/ResGH.png}} \\
\caption{Comparison between analytical PY solution of hard sphere static structure factor and thermodynamically consistent correction as described by Henderson and Grundke}
\label{fig:SQ:LHR}
\end{figure}
\clearpage
\section{Structure factor for a two dimensional hard spheres/disks fluid} \hspace{1pt}
The structure factor of hard disks or spheres with diameter/radius $\sigma=2R$ in two dimensions with an interaction potential
\begin{align}
U(r,\sigma) &=
U(r,\sigma) &=
\begin{cases}
\infty &\mathrm{for~} r < \sigma \\
0 &\mathrm{for~} r \geq \sigma
0 &\mathrm{for~} r \geq \sigma
\end{cases}
\end{align}
have been implemented in several variants.
@@ -33,10 +246,10 @@ \section{Structure factor for a two dimensional hard spheres/disks fluid} \hspac
with
\begin{align}
\begin{split}
c(r',\eta) &=
c(r',\eta) &=
\Theta(1-r') \left[-\frac{1-p\eta^2}{\left(1-2\eta+p\eta^2\right)^2}\right] \\
&\left\{1-a^2\eta-a^2\eta\frac{2}{\pi}\left[\arccos\left(\frac{r'}{a}\right)-\frac{r'}{a}\sqrt{1-\frac{r'^2}{a^2}}\right]\right\}
\end{split}
\end{split}
\end{align}
with $p=\left(4\sqrt{3}\pi-12\right)/\pi^2$ and $\Theta()$ being the Heaviside function. The parameter $a$ is the root of a non-linear equation, which has been solved numerically and then approximated via a polynomial fitting by
\begin{align}
@@ -323,7 +323,7 @@ \subsection{Broad-Peak}
\begin{center}
\includegraphics[width=0.85\textwidth,height=0.6\textwidth]{BroadPeak.png}
\end{center}
\caption{Emical form factor of Broad-Peak} \label{fig:BroadPeakIq}
\caption{Empirical form factor of Broad-Peak} \label{fig:BroadPeakIq}
\end{figure}
\clearpage
@@ -1122,7 +1122,7 @@ \subsubsection{P'(Q): wormlike PS2} ~\\
S_\mathrm{PS2}(Q,L,l_B) &=
\begin{cases}
S_\mathrm{SB}(Q,L,l_B) f_1 + S_\mathrm{loc}(Q,L,1) (1-f_1) & \mbox{ for~} n_b > 2 \\
S_\mathrm{Debye}(Q,Q^2\langle R_g^2\rangle_0) f_2 + S_\mathrm{loc}(Q,L,a_1) (1-f_2) & \mbox{ for~} n_b \leq 2
S_\mathrm{Debye}(Q,Q^2\langle R_g^2\rangle_0) f_2 + S_\mathrm{loc}(Q,L,l_B,a_1) (1-f_2) & \mbox{ for~} n_b \leq 2
\end{cases}
\end{align}
with $\langle R_g^2\rangle_0 = Ll_B/6\left(1-\frac{3}{2 n_b}
@@ -1147,7 +1147,7 @@ \subsubsection{P'(Q): wormlike PS2} ~\\
\end{align}
and in the large $Q$ range
\begin{align}
S_\mathrm{loc}(Q,L,a) &= \frac{a}{LbQ^2} + \frac{\pi}{LQ}
S_\mathrm{loc}(Q,L,l_B,a_1) &= \frac{a_1}{Ll_BQ^2} + \frac{\pi}{LQ}
\end{align}
The optimized values of the parameters $q_1,p_1, a_1, a_2, q_2$, and $ p_2$ are
$q_1 = 5.53$, $p_1 = 5.33$, $a_1 = 0.0625$, $a_2 = 11.7$, $p_2 = 3.95$ and
@@ -1175,27 +1175,27 @@ \subsubsection{P'(Q): wormlike PS2} ~\\
\subsubsection{P'(Q): wormlike PS3} ~\\
\label{plugin:Pprime4wormPS3}
This version of the wormlike structure model originally from \cite{Pedersen96Macrom} is implemented following the suggestions for corrections given in \cite{Chen2006}.
This version of the wormlike structure model originally from \cite{Pedersen96Macrom} is implemented together with the suggestions for corrections given in \cite{Chen2006}.
\begin{align}
q_0 &=
\begin{cases}
3.1 & \mbox{for~} L>4l_B \\
\max\left\{a_3 l_B/R_G,4\right\} & \mbox{for~} L\leq 4l_B \wedge \mbox{without excluded volume}\\
\max\left\{a_3 l_B/R_G,3\right\} & \mbox{for~} L\leq 4l_B \wedge \mbox{with excluded volume}\\
\end{cases} \\
\max\left\{a_3 l_B/R_G,4\right\} & \mbox{for~} L\leq 4l_B \wedge \mbox{without excl. vol.}\\
\max\left\{a_3 l_B/R_G,3\right\} & \mbox{for~} L\leq 4l_B \wedge \mbox{with excl. vol.}\\
\end{cases}\\
R_G^2 &=
\begin{cases}
\frac{Ll_B}{6} & \mbox{for~} L > 4l_B \wedge \mbox{without excluded volume} \\
\frac{Ll_B}{6} f\left(\frac{L}{l_B}\right) & \mbox{for~} L > 4l_B \wedge \mbox{with excluded volume} \\
\frac{Ll_B}{6} f\left(\frac{L}{l_B}\right)& \mbox{for~} L\leq 4l_B \wedge \mbox{without excluded volume}\\
\frac{Ll_B}{6} f\left(\frac{L}{l_B}\right)\alpha^2\left(\frac{L}{l_B}\right) & \mbox{for~} L\leq 4l_B \wedge \mbox{with excluded volume}\\
\frac{Ll_B}{6} & \mbox{for~} L > 4l_B \wedge \mbox{without excl. vol.} \\
\frac{Ll_B}{6} f\left(\frac{L}{l_B}\right) & \mbox{for~} L > 4l_B \wedge \mbox{with excl. vol.} \\
\frac{Ll_B}{6} f\left(\frac{L}{l_B}\right)& \mbox{for~} L\leq 4l_B \wedge \mbox{without excl. vol.}\\
\frac{Ll_B}{6} f\left(\frac{L}{l_B}\right)\alpha^2\left(\frac{L}{l_B}\right) & \mbox{for~} L\leq 4l_B \wedge \mbox{with excl. vol.}\\
\end{cases} \\
f(x) &= 1-\frac{3}{2x}+\frac{3}{2\left(x\right)^2}-\frac{3}{4\left(x\right)^3}\left[1-\exp\left(-2x\right)\right]\\
\alpha(x) &= \left(1+(x/3.12)^2+(x/8.67)^3\right)^{\epsilon/3} \\
\epsilon&=0.170 \\
C\left(\frac{L}{l_B}\right) &=
\begin{cases}
a_4/\left(\frac{L}{l_B}\right)^{p_3} & \mbox{for~} L > 10l_B \wedge \mbox{with excluded volume} \\
a_4/\left(\frac{L}{l_B}\right)^{p_3} & \mbox{for~} L > 10l_B \wedge \mbox{with excl. vol.} \\
1 & \mbox{ otherwise}\\
\end{cases}
\end{align}
@@ -1204,31 +1204,31 @@ \subsubsection{P'(Q): wormlike PS3} ~\\
S(Q,L,l_B) &=
\begin{cases}
L^2 S_{\textrm{small~} Q}(Q,L,l_B)& \mbox{for~} Ql_B<q_0 \\
L^2 \left(\frac{a_1}{(Ql_B)^{p_1}}+\frac{a_2}{(Ql_B)^{p_2}}+\frac{\pi}{QL}\right))& \mbox{for~} Ql_B\geq q_0 \\
L^2 \left(\frac{a_1}{(Ql_B)^{p_1}}+\frac{a_2}{(Ql_B)^{p_2}}+\frac{\pi}{QL}\right)& \mbox{for~} Ql_B\geq q_0 \\
\end{cases}\\
p_1 &=
\begin{cases}
4.95 & \mbox{for~} L > 4l_B \wedge \mbox{without excluded volume} \\
5.13 & \mbox{for~} L \leq 4l_B \wedge \mbox{without excluded volume} \\
4.12 & \mbox{for~} L > 4l_B \wedge \mbox{with excluded volume} \\
5.36 & \mbox{for~} L \leq 4l_B \wedge \mbox{with excluded volume} \\
4.95 & \mbox{for~} L > 4l_B \wedge \mbox{without excl. vol.} \\
5.13 & \mbox{for~} L \leq 4l_B \wedge \mbox{without excl. vol.} \\
4.12 & \mbox{for~} L > 4l_B \wedge \mbox{with excl. vol.} \\
5.36 & \mbox{for~} L \leq 4l_B \wedge \mbox{with excl. vol.} \\
\end{cases}\\
p_2 &=
\begin{cases}
5.29 & \mbox{for~} L > 4l_B \wedge \mbox{without excluded volume} \\
7.47 & \mbox{for~} L \leq 4l_B \wedge \mbox{without excluded volume} \\
4.42 & \mbox{for~} L > 4l_B \wedge \mbox{with excluded volume} \\
5.62 & \mbox{for~} L \leq 4l_B \wedge \mbox{with excluded volume} \\
5.29 & \mbox{for~} L > 4l_B \wedge \mbox{without excl. vol.} \\
7.47 & \mbox{for~} L \leq 4l_B \wedge \mbox{without excl. vol.} \\
4.42 & \mbox{for~} L > 4l_B \wedge \mbox{with excl. vol.} \\
5.62 & \mbox{for~} L \leq 4l_B \wedge \mbox{with excl. vol.} \\
\end{cases}
\end{align}
The parameters $a_1$ and $a_2$ are defined by the condition that for $Ql_B=q_0$ the small and large $Q$ region are continuous and smooth, i.e. that also the first derivative of $\mathrm{d}S(q,L,l_B)/\mathrm{d}q|_{q=q_0/l_B}$ is continuous.
\begin{align}
S_{\textrm{small~} Q}(Q,L,l_B) &=
\begin{cases}
S_\textrm{SB}(Q,R_G) & \mbox{for~} L > 4l_B \\
S_\textrm{Debye}(Q,R_G) & \mbox{for~} L \leq 4l_B \wedge \mbox{without excluded volume}\\
S_\textrm{cexv}(Q,R_G) & \mbox{for~} L \leq 4l_B \wedge \mbox{with excluded volume} \\
\end{cases}\\
S_\textrm{Debye}(Q,R_G) & \mbox{for~} L \leq 4l_B \wedge \mbox{without excl. vol.}\\
S_\textrm{cexv}(Q,R_G) & \mbox{for~} L \leq 4l_B \wedge \mbox{with excl. vol.} \\
\end{cases}
\end{align}
where
\begin{align}
@@ -1240,7 +1240,7 @@ \subsubsection{P'(Q): wormlike PS3} ~\\
x &= QR_G \\
S_\textrm{EXV}(Q,R_G) &= (1-w)S_\textrm{Debye}(Q,R_G) \\
&+ w f_\textrm{corr}(Q) \left(C_1x^{-1/\nu}+C_2x^{-2/\nu}+C_3x^{-3/\nu}\right) \nonumber \\
w &= \frac12\left(1+\tanh((x-1.523)/0.1477)\right) \\
w &= \frac12\left(1+\tanh((x-1.523)/0.1477)\right)
\end{align}
with the optimized parameters $C_1 = 1.2220$, $C_2 = 0.4288$, and $C_3 = -1.651$.
As $S_\textrm{EXV}$ should be a monotonic decreasing function also at very small $Q$-values the correction factor $f_\textrm{corr}(Q,R_G)$ has been introduced by \cite{Chen2006}
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