Ab initiostudies of diamond(100) surface reconstruction

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Abstract
The reconstruction of the C(100) surface is studied by a cluster model at several theoretical levels. It is found that the calculated surface-dimer bond length is 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 (SCF) treatment gives a closed-shell singlet state, higher in energy than the triplet state, and with a dimer length of 1.401 Å, 0.279 Å shorter than the triplet. The dimer is found to be symmetric for both singlet and triplet states at the SCF and complete active space (2,2) levels of theory. The correct ground state is a singlet, but a multideterminant wave function is required for its description. At the configuration-interaction level, the surface-dimer bond length in the ground state is found to be 1.508 Å and the energy decrease on dimer formation with respect to the ideal C(100) 1×1 surface is 2.28 eV per dimer.