Abstract Reconstruction on (111) Si and diamond surfaces is investigated by employing the MINDO/3 quantum chemistry procedure to optimise the geometries of clusters representing these surfaces. Localised molecular orbitals are constructed to interpret the reconstruction mechanisms in terms of bond changes. The clusters consist of eight Si or C atoms together with bond saturating hydrogens. Since two of the eight Si/C atoms represent first layer atoms, the cluster can spontaneously discriminate between these two atoms and therefore both 1 × 1 and 2 × 1 reconstruction can be modelled. It is found that the Si cluster indeed experiences an asymmetrical distortion in which one atom is raised with respect to the surface while the other is lowered. These displacements are accompanied by asymmetric dehybridisation of surface bonds and a transfer of charge between the two surface atoms as expected for such a Jahn-Teller distortion. Intra-atomic Coulomb repulsion in the raised, negatively charged atom is shown to be an important factor in opposing the reconstruction. In contrast to the Si cluster, the diamond cluster hardly shows any buckling, but considerable relaxation occurs. The structure does not differ substantially from a 1 × 1 reconstructed configuration. The reluctance of the diamond surface to undergo a Jahn-Teller distortion is ascribed to partial graphitisation of this surface.