The bond of the silicon atoms 1 and 2 does not persist.
Instead the silicon atom forms a bond with the initial carbon interstitial and the second carbon atom forms a bond with silicon atom 1 forming four bonds in total.
The carbon atoms are spaced by 3.14 \AA, which is very close to the expected C-C next neighbour distance of 3.08 \AA{} in silicon carbide.
+Figure \ref{fig:defects:comb_db_02} c) displays the results of another \hkl<0 0 1> dumbbell inserted at position 3.
+The binding energy is -2.05 eV.
+Both dumbbells are tilted along the same direction remaining parallely aligned and the second dumbbell is pushed downwards in such a way, that the four dumbbell atoms form a rhomboid.
+Both carbon atoms form tetrahedral bonds to four silicon atoms.
+However, silicon atom 1 and 3, which are bond to the second carbon dumbbell interstitial are also bond to the initial carbon atom.
+These four atoms of the rhomboid reside in a plane and, thus, do not match the situation in silicon carbide.
+The carbon atoms have a distance of 2.75 \AA.
+In figure \ref{fig:defects:comb_db_02} b) a second \hkl<0 1 0> dumbbell is constructed at position 2.
+An energy of -1.90 eV is observed.
+The initial dumbbell and especially the carbon atom is pushed towards the silicon atom of the second dumbbell forming an additional fourth bond.
+Silicon atom number 1 is pulled towards the carbon atoms of the dumbbells accompanied by the disappearance of its bond to silicon number 5 as well as the bond of silicon number 5 to its next neighboured silicon atom in \hkl<1 1 -1> direction.
+The carbon atom of the second dumbbell forms threefold coordinated bonds to its silicon neighbours.
+A distance of 2.80 \AA{} is observed for the two carbon atoms.
+Again, the two carbon atoms and its two interconnecting silicon atoms form a rhomboid.
+C-C distances of 2.70 to 2.80 \AA{} seem to be characteristic for such configurations, in which the carbon atoms and the two interconnecting silicon atoms reside in a plane.
+
+Configurations obtained by adding a second dumbbell interstitial at position 4 are characterized by minimal changes from their initial creation condition during relaxation.
+There is a low interaction of the dumbbells, which seem to exist independent of each other.
+This, on the one hand, becomes evident by investigating the final structure, in which both of the dumbbells essentially retain the structure expected for a single dumbbell and on the other hand is supported by the observed binding energies which vary only slightly around zero.
+This low interaction is due to the larger distance and a missing direct connection by bonds along a crystallographic direction.
+Both carbon and silicon atoms of the dumbbells form threefold coordinated bonds to their next neighbours.
+The energetically most unfavorable configuration ($E_{\text{b}}=0.26\text{ eV}$) is obtained for the \hkl<0 0 1> interstitial oppositely orientated to the initial one.
+A dumbbell taking the same orientation as the initial one is less unfavorble ($E_{\text{b}}=0.04\text{ eV}$).
+Both configurations are unfavorable compared to far-off isolated dumbbells.
+Nonparallel orientations, that is the \hkl<0 1 0>, \hkl<0 -1 0> and its equivalents, result in binding energies of -0.12 eV and -0.27 eV, thus, constituting energetically favorable configurations.
+The reduction of strain energy is higher in the second case where the carbon atom of the second dumbbell is placed in the direction pointing away from the initial carbon atom.
+\begin{figure}[h]
+\begin{center}
+\begin{minipage}[t]{7cm}
+a) \underline{$E_{\text{b}}=-1.53\text{ eV}$}
+\begin{center}
+\includegraphics[width=6.0cm]{00-1dc/1-53.eps}
+\end{center}
+\end{minipage}
+\begin{minipage}[t]{7cm}
+b) \underline{$E_{\text{b}}=-1.66\text{ eV}$}
+\begin{center}
+\includegraphics[width=6.0cm]{00-1dc/1-66.eps}
+\end{center}
+\end{minipage}\\[0.2cm]
+\begin{minipage}[t]{7cm}
+c) \underline{$E_{\text{b}}=-1.88\text{ eV}$}
+\begin{center}
+\includegraphics[width=6.0cm]{00-1dc/1-88.eps}
+\end{center}
+\end{minipage}
+\begin{minipage}[t]{7cm}
+d) \underline{$E_{\text{b}}=-1.38\text{ eV}$}
+\begin{center}
+\includegraphics[width=6.0cm]{00-1dc/1-38.eps}
+\end{center}
+\end{minipage}
+\end{center}
+\caption{Relaxed structures of defect complexes obtained by creating a a) \hkl<0 0 1>, a b) \hkl<0 0 -1>, a c) \hkl<0 -1 0> and a d) \hkl<1 0 0> dumbbell at position 5.}
+\label{fig:defects:comb_db_05}
+\end{figure}
+Energetically beneficial configurations of defect complexes are observed for second interstititals of all orientations placed at position 5, a position two bonds away from the initial interstitial along the \hkl<1 1 0> direction.
+Relaxed structures of these complexes are displayed in figure \ref{fig:defects:comb_db_05}.
+Figure \ref{fig:defects:comb_db_05} a) and b) show the relaxed structures of \hkl<0 0 1> and \hkl<0 0 -1> dumbbells.
+The upper dumbbell atoms are pushed towards each other forming fourfold coordinated bonds.
+While the displacements of the silicon atoms in case b) are symmetric to the \hkl(1 1 0) plane, in case a) the silicon atom of the initial dumbbel is pushed a little further in the direction of the carbon atom of the second dumbbell than the carbon atom is pushed towards the silicon atom.
+The bottom atoms of the dumbbells remain in threefold coordination.
+The symmetric configuration is energetically more favorable ($E_{\text{b}}=-1.66\text{ eV}$) since the displacements of the atoms is less than in the antiparallel case ($E_{\text{b}}=-1.53\text{ eV}$).
--2.05 ... both C atoms correctly coordinated, however (check C-C distance, too close?) wrong coordination of the C-Si-C bonds which reside in a plane ... all the 4 participating atoms reside in a plane ...
-
--1.90 ... again, one C atom bound to 4 Si atoms but the second one only bond to three Si atoms. However, C-C slighlty higher.
-
-The 2.7 diatnce characteristic for configurations with one C atom bound to 4 silicon and.
-Different energies due to slightly varying constellation.
+... typical C-C distance 2.79 \AA ...
-the more far-off ones:
--0.27 and -0.12 ... carbon is threefold coordinated.
-initial structures evident, seem to be independent of each other ...
+I
but better ...
-1.88 and -1.38 ...
-1.38: Si (I) moves towards 2nd Si int in 110 direction, such that both Si 4fold coordinated and C remain 3fold ...
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