) where n Iσ m are localised orbital occupation numbers with atomic site index I, state index m, and spin σ . n Iσ m is calculated as the projection of occupied Kohn-Sham DFT orbitals on a localised basis set. U eff is set in CASTEP as a parameter that applies to all the orbitals within a given subshell (e.g. d-subshell).