  Grain boundaries (GBs) play a fundamental role in the strengthening mechanism
of crystalline structures by acting as an impediment to dislocation motion.
However, the presence of an aggressive environment such as hydrogen increases
the susceptibility to intergranular fracture. Further, there is a lack of
systematic investigations exploring the role of hydrogen on the
dislocation-grain-boundary (DGB) interactions. Thus, in this work, the effect
of hydrogen on the interactions between a screw dislocation and <111> tilt GBs
in {\alpha}-Fe were examined. Our simulations reveal that the outcome of the
DGB interaction strongly depends on the underlying GB dislocation network.
Further, there exists a strong correlation between the GB energy and the energy
barrier for slip transmission. In other words, GBs with lower interfacial
energy demonstrate a higher barrier for slip transmission. The introduction of
hydrogen along the GB causes the energy barrier for slip transmission to
increase consistently for all of the GBs examined. The energy balance for a
crack initiation in the presence of hydrogen was examined with the help of our
observations and previous findings. It was found that the presence of hydrogen
increases the strain energy stored within the GB which could lead to a
transgranular-to-intergranular fracture mode transition.
