Difference in formation of hydrogen and helium clusters in tungsten

The experimentally observed large difference in the depths of hydrogen and helium clusters formed in tungsten still lacks a fundamental explanation. Using density functional theory calculations, molecular dynamics simulations, and kinetic Monte Carlo calculations, we show that the fundamental mechanism behind the different clustering depths is significantly different behaviors of interstitial H and He atoms in W: H–H states are unstable for small interatomic distances whereas He–He states are strongly bound.