  The oxygen self-diffusion coefficient in UO${_2}$ has been recently studied
[Cooper et al. Solid State Ionics 282 (2015) 26-30] over a range of pressures
(0-10GPa) and temperatures (300-1900K) by combining molecular dynamics
calculations with a thermodynamical model, the cB{\Omega} model. A significant
reduction in oxygen self-diffusion as a function of increasing hydrostatic
pressure, and the associated increase in activation energy was identified.
Here, we extend this study and find that the compressibility of the
corresponding activation volume exceeds significantly the compressibility of
the bulk material by almost one order of magnitude. This results is important
since in the literature it is usually assumed that these two compressibilities
are equal. The same holds when comparing the thermal expansion coefficient of
this volume with that of the bulk solid.
