The reconstruction of the Si(100) surface was studied by a cluster method at several theoretical levels. It was found that the calculated surface dimer bond length was very sensitive to the level of theoretical treatment. A single-determinant SCF treatment gives a closed-shell singlet state higher in energy than the triplet state and a dimer length for the singlet of 2.21Å; 0.26Å shorter than the triplet. The correct ground state was a singlet, but a multi-determinant wave-function was required for its description. At the CI level, the surface dimer in the ground state was found to be symmetrical with a dimer bond length of 2.40Å and the energy decrease for dimer formation with respect to the ideal Si(100)-(1 x 1) surface was 1.98eV per dimer. The total energies of 10 alternative structural models were also calculated. It was found that, in general, those models with very short dimer lengths (< 2.25Å) and /or with large buckling were less stable than those models with longer dimer lengths (~ 2.40Å) and with little or no buckling.

Ab initio Studies of Si(100) Surface Reconstruction. Z.Jing, J.L.Whitten: Surface Science, 1992, 274[1], 106-12