The H pair defects in the bulk, and at the (111) surface, were investigated by using hybrid density functional theory and cluster models; including relaxation of defect atoms. The H2* defect, which consisted of one H atom in a bond inserted site and the other in an adjacent anti-bonding site, was calculated to be 2.65eV more stable than 2 isolated bond inserted H atoms, and 3.97eV less stable than 2 gas-phase H atoms. This result was in general agreement with the results found by using other reported methods. However, when the anti-bonding H atom was at the surface, and the bond inserted H was just beneath it, the energy with respect to 2 gas-phase H atoms was more stable by 0.95eV and was stable by 7.57eV relative to 2 bulk bond inserted H atoms. The H atom bond to the surface had a strength of 3.89eV. Such structures had not previously been reported, and could contribute to the excess H found at diamond surfaces; thus providing a route to H2* formation in the bulk.

Hydrogen Atom Pairs in Diamond Bulk and at the Surface - Hybrid Density Functional Theory and Cluster Models. A.B.Anderson, L.N.Kostadinov, J.C.Angus: Physical Review B, 2003, 67[23], 233402