The reconstruction of the C(100) surface was studied by a cluster model at several theoretical levels. It was found that the calculated surface-dimer bond length was very sensitive to the level of theoretical treatment, the spin state, and the degree of constraints in the geometry optimization process. A single-determinant self-consistent field treatment gives a closed-shell singlet state, higher in energy than the triplet state, and with a dimer length of 1.401Å; 0.279Å shorter than the triplet. The dimer was found to be symmetric for both singlet and triplet states at the self-consistent field and complete active space (2,2) levels of theory. The correct ground state was a singlet, but a multi-determinant wave function was required for its description. At the configuration-interaction level, the surface-dimer bond length in the ground state was found to be 1.508Å and the energy decrease on dimer formation with respect to the ideal C(100)-(1 x 1) surface was 2.28eV/dimer.

Ab initio Studies of Diamond(100) Surface Reconstruction. Z.Jing, J.L.Whitten: Physical Review B, 1994, 50, 2598-605