  The different point defects formed by two hydrogen atoms or two helium atoms
in tungsten were investigated through first-principles calculation. The
energetically favorable site for a hydrogen atom is tetrahedral interstitial
site while substitutional site is the most preferred site for a helium atom.
The formation energies of two hydrogen or helium atoms are determined by their
positions, and there are not a multiple of a single atom's. After relaxation,
two adjacent hydrogen atoms are away from each other while helium atoms are
close to. The reasons for the interaction between two hydrogen or helium atoms
are also discussed.
