The weakening of the interstitial hydrogen molecule bond, due to screening effects from the surrounding lattice, might be expected to have an effect on the barrier to the molecule passing through a hexagonal site on its diffusion path through the crystal. If the charge density within the lattice was severely affected by excess carrier generated by, for example irradiation, then the diffusion barrier would be changed, and this is indeed what we find. The diffusion barrier drops from 0.73 eV in the neutral case, to 0.33 and 0.46 eV for clusters charged to +2 or -2 respectively, with an expansion of the bond length whilst the molecule is at the the hexagonal site from 0.788 Å to 0.901 and 0.930 Å respectively (The bond length at the tetrahedral site is insensitive to charge state).