  Despite well documented first-principles theoretical determination of the low
migration energy (0.06 eV) of a single He in tungsten, fully quantum mechanical
calculations on the migration of a He pair still present a challenge due to the
complexity of its trajectory. By identifying the six most stable configurations
of the He pair in W and decomposing its motion into rotational, translational,
and rotational-translational routines, we are able to determine its migration
barrier and trajectory. Our density functional theory calculations demonstrate
a He pair has three modes of motion: a close or open circular two-dimensional
motion in (100) plane with an energy barrier of 0.30 eV, a snaking motion along
[001] direction with a barrier of 0.30 eV, and a twisted-ladder motion along
[010] direction with the two He swinging in the plane (100) and a barrier of
0.31 eV. The graceful associative movements of a He pair are related to the
chemical-bonding-like He-He interaction being much stronger than its migration
barrier in W. The excellent agreement with available experimental measurements
(0.24-0.32 eV) on He migration makes our first-principles result a solid input
to obtain accurate He-W interatomic potentials in molecular dynamics
simulations.
