  Density Functional Theory (DFT) calculations show a weak interaction between
hydrogen and helium in iron, in contrast to previous reports of a strong
trapping of hydrogen at helium. The strong preference of He and H to occupy
regions with low electronic density (such as vacancies) explains this
discrepancy, with vacancy-He and vacancy-H binding forces concealing the
repulsive interaction between He and H. Furthermore, Rate Theory simulations
based on our DFT-calculated V$_n$He$_m$H$_p$ cluster energetics predict, as it
is observed in some experiments, that synergetic effects could be expected
between H and He in iron under irradiation.
