  First-principles atomistic molecular dynamics simulation in the
micro-canonical and canonical ensembles has been used to study the diffusion of
interstitial hydrogen in $\alpha$-iron. Hydrogen to Iron ratios between
$\theta=1/16 and 1/2 have been considered by locating interstitial hydrogen
atoms at random positions in a $2 \times 2 \times 2$ supercell. We find that
the average optimum absorption site and the barrier for diffusion depend on the
concentration of interestitials. Iron Debye temperature decreases monotonically
for increasing concentration of interstitial hydrogen, proving that iron-iron
interatomic potential is significantly weakened in the presence of a large
number of diffusing hydrogen atoms.
