Atomistic simulations of the effects of H on edge dislocation mobility and pile-ups are performed to investigate possible nanoscale mechanisms for hydrogen-enhanced localized plasticity (HELP). alpha-Fe is used as a model system because H diffusion is fast enough to capture kinetics within the time scales of molecular dynamics and because edge dislocation glide in alpha-Fe is similar to glide in face-centered cubic metals. Results over a wide range of H concentrations sufficient to generate a range of sizes in the Cottrell atmospheres show that the Cottrell atmospheres follow the moving dislocations, leading to a resistance to dislocation motion that is consistent with solute drag theory; thus, H reduces the dislocation mobility. Furthermore, once motion stops and a pile-up is established, the H Cottrell atmospheres do not affect the equilibrium spacing of dislocations in the pile-up; thus, the H atmosphere provides no "shielding" of dislocation dislocation interactions. This result is consistent with conclusions from previous continuum calculations. Two oft-proposed mechanisms for HELP (H-induced increase in true dislocation mobility and decrease in dislocation-dislocation pile-up interactions) are therefore not supported by the present simulations. A mechanistic understanding of HELP phenomena observed in various experiments thus requires evaluation of more complex H-dislocation interactions. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved