We present theoretical studies of relaxations of monoatomic and steps on the diamond(100)-(21) surface employing an ab initio molecular dynamics simulation method that is based on density functional theory. Stable dimer structures are found in the upper and lower planes of the step surfaces in agreement with experiment. Significant atomic relaxations occur near the step edges of and stepped surfaces induced by the creation of the steps. Atomic H adsorption on these step surfaces to form monohydride structures is energetically favourable. We also simulate the presence of radical sites near the step edges of , and and local reconstruction involving the dimer containing the radical sites is found. Electronic charge density profiles of the filled states near the Fermi level show features associated with the dimer structures.