+\section{Mobility of the silicon self-interstitial}
+
+The possibility for separated configurations of \cs{} and \si{} becomes even more likely if one of the constituents exhibits a low barrier of migration.
+In this case, the \si{} is assumed to constitute the mobile defect compared to the stable \cs{} atom.
+Thus, migration paths of \si{} are investigated in the following excursus.
+According to Fig.~\ref{fig:defects:si_mig1}, an activation energy of \unit[0.67]{eV} is necessary for the transition of the \si{} \hkl[0 -1 1] to \hkl[1 1 0] DB located at the neighbored Si lattice site in \hkl[1 1 -1] direction.
+\begin{figure}[tp]
+\begin{center}
+\includegraphics[width=0.7\textwidth]{si_110_110_mig_02_conf.ps}
+\end{center}
+\caption[Migration barrier and structures of the \si{} {\hkl[0 -1 1]} DB to the {\hkl[1 1 0]} DB transition.]{Migration barrier and structures of the \si{} \hkl[0 -1 1] DB (left) to the \hkl[1 1 0] DB (right) transition. Bonds are illustrated by blue lines.}
+\label{fig:defects:si_mig1}
+\end{figure}
+The barrier, which is even lower than the one for \ci{}, indeed indicates highly mobile \si.
+In fact, a similar transition is expected if the \si{} atom, which does not change the lattice site during transition, is located next to a \cs{} atom.
+Due to the low barrier the initial separation of the \cs{} and \si{} atom are very likely to occur.
+Further investigations revealed transition barriers of \unit[0.94]{eV} for the Si$_{\text{i}}$ \hkl[1 1 0] DB to the hexagonal Si$_{\text{i}}$, \unit[0.53]{eV} for the Si$_{\text{i}}$ \hkl[1 1 0] DB to the tetrahedral Si$_{\text{i}}$ and \unit[0.35]{eV} for the hexagonal Si$_{\text{i}}$ to the tetrahedral Si$_{\text{i}}$ configuration.
+The respective configurational energies are shown in Fig.~\ref{fig:defects:si_mig2}.
+\begin{figure}[tp]
+\begin{center}
+\includegraphics[width=0.7\textwidth]{si_mig_rest.ps}
+\end{center}
+\caption[Migration barrier of the \si{} {\hkl[1 1 0]} DB into the hexagonal and tetrahedral configuration as well as the hexagonal \si{} to tetrahedral \si{} transition.]{Migration barrier of the \si{} \hkl[1 1 0] DB into the hexagonal (H) and tetrahedral (T) configuration as well as the hexagonal \si{} to tetrahedral \si{} transition.}
+\label{fig:defects:si_mig2}
+\end{figure}
+The obtained activation energies are of the same order of magnitude than values derived from other {\em ab initio} studies \cite{bloechl93,sahli05}.
+The low barriers indeed enable configurations of further separated \cs{} and \si{} atoms by the highly mobile \si{} atom departing from the \cs{} defect as observed in the previously discussed MD simulation.
+