  The self-interaction corrected (SIC) local spin-density approximation (LSD)
is used to investigate the groundstate valency configuration of the actinide
ions in the actinide mono-carbides, AC (A = U, Np, Pu, Am, Cm), and the
actinide mono-nitrides, AN. The electronic structure is characterized by a
gradually increasing degree of f-electron localization from U to Cm, with the
tendency towards localization being slightly stronger in the (more ionic)
nitrides compared to the (more covalent) carbides. The itinerant band-picture
is found to be adequate for UC and acceptable for UN, whilst a more complex
manifold of competing localized and delocalized f-electron configurations
underlies the groundstates of NpC, PuC, AmC, NpN, and PuN. The fully localized
5f-electron configuration is realized in CmC (f7), CmN (f7), and AmN (f6). The
observed sudden increase in lattice parameter from PuN to AmN is found to be
related to the localization transition. The calculated valence electron
densities of states are in good agreement with photoemission data.
