  We study the effects of hydrostatic pressure in the range 0.0--2.0 GPa on
anion mobility in the orthorhombic $Pnma$ phase of CsPbBr$_{3}$. Using density
functional theory and the climbing nudged elastic band method, we calculate the
transition states and activation energies for anions to migrate both within and
between neighbouring PbBr$_{3}$ octahedra. The results of those calculations
are used as input to a kinetic model for anion migration, which we solve in the
steady state to determine the anion mobility tensor as a function of applied
pressure. We find that the response of the mobility tensor to increasing
pressure is highly anisotropic, being strongly enhanced in the $(010)$ lattice
plane and strongly reduced in the direction normal to it at elevated pressure.
These results demonstrate the potentially significant influence of pressure and
strain on the magnitude and direction of anion migration in lead--halide
perovskites.
