Structure of the Si(100)2×1 surface: Total-energy and force analysis of the dimer models

Results of extensive studies of the dimer model of the reconstructed Si(100)2×1 surface with molecular clusters containing up to 63 atoms are reported. An all-electron numerical method based on the local-density-functional approach for molecules with analytic energy gradients is applied to both symmetric and asymmetric dimer models. By calculating the force on the first four silicon layers and minimizing the total energy of the cluster, the optimized atomic geometry with minimum energy is obtained. The first-layer Si atoms are found to relax inward by about 0.38 for the symmetric dimer, and 0.16 and 0.55 for the asymmetric dimers. The dimer bond length is 2.23 and 2.27 for the symmetric and the asymmetric dimer, respectively. Further, the calculated energy difference between the symmetric and the asymmetric dimers is very small (0.02 eV); thus it is quite possible that both dimers could coexist on the surface.