  Hydrogen radical attacks and subsequent hydrogen migrations are considered to
play an important role in the atomic-scale mechanisms of diamond chemical
vapour deposition growth. We perform a comprehensive analysis of the reactions
involving H-radical and vacancies on H-passivated diamond surfaces exposed to
hydrogen radical-rich atmosphere. By means of first principles calculations --
density functional theory and climbing image nudged elastic band method --
transition states related to these mechanisms are identified and characterised.
In addition, accurate reaction rates are computed using variational transition
state theory. Together, these methods provide -- for a broad range of
temperatures and hydrogen radical concentrations -- a picture of the relative
likelihood of the migration or radical attack processes, along with a
statistical description of the hydrogen coverage fraction of the (100)
H-passivated surface, refining earlier results via a more thorough analysis of
the processes at stake. Additionally, the migration of H-vacancy is shown to be
anisotropic, and occurring preferentially across the dimer rows of the
reconstructed surface. The approach used in this work can be generalised to
other crystallographic orientations of diamond surfaces or other
semiconductors.
