+The silicon atom numbered '1' and the C atom compose the dumbbell structure.
+They share the lattice site which is indicated by the dashed red circle and which they are displaced from by length $a$ and $b$ respectively.
+The atoms no longer have four tetrahedral bonds to the silicon atoms located on the alternating opposite edges of the cube.
+Instead, each of the dumbbell atoms forms threefold coordinated bonds, whcih are located in a plane.
+One bond is formed to the other dumbbell atom.
+The other two bonds are bonds to the two silicon edge atoms located in the opposite direction of the dumbbell atom.
+Angles ...
+The distance of the two dumbbell atoms is almost the same for both types of calculations.