  We investigate the effect of hydrogen on the mobility of a screw dislocation
in body-centered cubic (bcc) iron using first-principles calculations, and show
that an increase of screw dislocation velocity is expected for a limited
temperature range. The interaction energy between a screw dislocation and
hydrogen atoms is calculated for various hydrogen positions and dislocation
configurations with careful estimations of the finite size effects, and the
strongest binding energy of a hydrogen atom to the stable screw dislocation
configuration is estimated to be $256\pm32$ meV. These results are incorporated
into a line tension model of a curved dislocation line to elucidate the effect
of hydrogen on the dislocation migration process. Both the softening and
hardening effect of hydrogen, caused by the reduction of kink nucleation
enthalpy and kink trapping, respectively, are evaluated. A clear transition
between softening and hardening behavior at the lower critical temperature is
predicted, which is in qualitative agreement with the experimental observation.
