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We use ab initio local-density-approximation methods to study the electronic and geometrical structure of the various C (100) surfaces. The calculated densities of states suggest that the surface states observed for the (2× 1) surface are attributable only to the clean C (100)(2× 1), not the C (100)(2× 1): H. The hypothetical C (100)(1× 1): 2H is found to be energetically unstable. The C (100)(3× 1): 1.33 H is found to be favored over the C (100)(2× 1): H. H desorption energies are calculated.