+Substitutional carbon (C$_{\text{sub}}$) in silicon, which is in fact not an interstitial defect, is found to be the lowest configuration with regard to energy for all potential models.\r
+VASP calculations performed in this work are in good agreement with results obtained by classical potential simulations by Tersoff\cite{tersoff90} and ab initio calculations done by Dal Pino et~al\cite{dal_pino93}.\r
+However, the Erhart/Albe potential dramatically underestimtes the C$_{\text{sub}}$ formation energy, which is a definite drawback of the potential.\r
+\r
+Except for the Tersoff potential the \hkl<1 0 0> dumbbell is the energetically most favorable interstital configuration, in which the C and Si dumbbell atoms share a Si lattice site.\r
+This finding is in agreement with several theoretical\cite{burnard93,leary97,dal_pino93,capaz94} and experimental\cite{watkins76,song90} investigations.\r
+Tersoff as well, considers the \hkl<1 0 0> to be the ground state configuration and believes an artifact due to the abrupt C-Si cut-off used in the potential to be responsible for the small value of the tetrahedral formation energy\cite{tersoff90}.\r
+A qualitative difference is observed investigating the dumbbell structures.\r
+While the C-Si distance of the dumbbell atoms is almost equal for both methods, the vertical position of the dumbbell inside the tetrahedra spanned by the four next neighboured Si atoms differs significantly.\r
+The dumbbell based on the Erhart/Albe potential is almost centered around the regular Si lattice site as can be seen in Fig. \ref{fig:defects} whereas for VASP calculations it is translated upwards with the C atom forming an almost collinear bond to the two Si atoms of the top face of the tetrahedra and the bond angle of the Si dumbbell atom to the two bottom face Si atoms approaching \unit[120]{$^\circ$}.\r
+This indicates predominant sp and sp$^2$ hybridization for the C and Si dumbbell atom respectively.\r
+Obviously the classical potential is not able to reproduce the clearly quantum-mechanical character of bonding.\r
+% empirical potential adjusts according to minimum of angular function, no QM!\r
+\r
+\r
+% pick up again later, that this is why erhart/albe is more promising for the specific problem under investigation\r
+\r
+\r
+\r