  Understanding helium accumulation in plasma-facing or structural materials in
a fusion reactor starts from uncovering the details of the migration of single
and paired He interstitials. We have carried out a first-principles density
functional theory investigation into the migration of both a single
interstitial He atom and an interstitial He-pair in bcc (Fe, Mo and W) and fcc
(Cu, Pd and Pt) metals. By identifying the most stable configurations of an
interstitial He-pair in each metal and decomposing its motion into rotational,
translational, and rotational-translational routines, we are able to determine
its migration barrier and trajectory. Our first-principles calculations reveal
that the migration trajectories and barriers are determined predominantly by
the relatively stable He-pair configurations which depend mainly on the
stability of a single He in different interstices. Contrary to atomistic
studies reported in literature, the migration barrier in bcc Fe, Mo, and W is
0.07, 0.07, and 0.08 eV respectively, always slightly higher than for a single
interstitial He (0.06 eV for all three). Configurations of a He-pair in fcc
metals are much more complicated, due to the stability closeness of different
interstitial sites for a single He atom. In both Cu and Pd, the migration of a
He-pair proceeds by moving one He at a time from one tetrahedral site to
neighboring octahedral site; whereas in Pt the two He move simultaneously
because the bridge interstitial site presents an extremely low barrier. The
migration barrier for a He-pair is 0.05, 0.15, and 0.04 eV for Cu, Pd, and Pt,
slightly lower than (in Cu), or similar to (in Pd and Pt) a single He, which is
0.08, 0.15, and 0.03 eV, respectively. The associative motions of a He-pair are
ensured by the strong He-He interactions in metals which are chemical
bonding-like and can be described very well with Morse potentials.
