Ab initio calculations have been performed to investigate the initial stage of the CVD growth of the diamond (100) surface with CH3 and H radicals. The reaction and the activation energies were calculated for nucleation on a flat terrace, the so-called dimer mechanism. CH2 insertion is found to be the rate-determining step of the dimer mechanism. CVD growth via the dimer mechanism is thermodynamically favorable, however, it is expected to be slow due to the high activation barrier. (c) 2005 Elsevier B.V. All rights reserved.