The role of a bonding in the (2 x 1) reconstruction of Si (111) was examined. The difference in total energy (ΔE) between the planar and twisted Si---Si π systems in a single π bond (Si2H4), a pair of π bonds (Si4H6) and an Si6 chain with three π bonds embedded in an Si12 cluster show a definite trend: ΔE decreases as the number of π bonds increases. For the largest system studied, an Si12 cluster model of the Si (111) surface, ΔE was negligibly small. The corresponding ΔE values for C2H4 and for models of the (2 x 1) reconstruction of the diamond (111) surface were larger than the values for silicon. Such differences between silicon and carbon could be understood in terms of smaller energy needed for atomic rehybridization (s2p2 → sp3 transition) for silicon. The results suggest that the partial breaking of Si---Si π bonds did not energetically inhibit surface reconstruction. Surface π Bonding in the (2 x 1) Reconstruction of Si (111). A.Chattopadhyay, P.V.Madhavan, J.L.Whitten, C.R.Fischer, I.P.Batra: Journal of Molecular Structure, 1988, 163, 63-78