(color online) Migration of interstitial hydrogen: (a) the NN interstitial sites C (green spheres) form a regular hexagon, located in the vicinity of W atoms (blue spheres). Interstitial sites G (yellow spheres) are located in the middle of the NNN interstitial site C. Note that we only represent W atoms by blue spheres for observation convenience. (b) Migration energy profiles for hydrogen migration in Be₂₂W are estimated using NEB calculation. The C2 site is the NN site of interstitial site C1 with a distance of 1.73 Å, and C3 is the NNN site of site C2 with a distance of 2.04 Å.