The electronic structure of the rare earth nitrides is studied systematically using the {it ab-initio} self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both a localised description of the rare earth $f-$electrons and an itinerant description of the valence electrons. Localising different numbers of $f$-electrons on the rare earth atom corresponds to different valencies, and the total energies can be compared, providing a first-principles description of valence. CeN is found to be tetravalent while the remaining rare earth nitrides are found to be trivalent. We show that these materials have a broad range of electronic properties including forming a new class of half-metallic magnets with high magnetic moments and are strong candidates for applications in spintronic and spin-filtering devices.
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