Local-density total energy calculations within the parameterfree self-consistent augmented-spherical-wave method are performed on ordered nonstoichiometric rock-salt-type structures of (\mathrm{VC}{0.75}) and (\mathrm{VN}{0.75}). Starting from the idea of two kinds of building Inits, (M_{t} X) and (M_{t}\left[{ }^{-}\right](M, X), and ([) ' are the transition metal, a nommetal, and the vacancy, respectively), we investigate interactions of octahedral bonds formed by the octahedral arrangement of transition metals (M_{6} \square). Using enlarged unit cells with the same composition we have found ordered structures with contacting corners of octahedra (corner-sharing structures) energetically favorable for both carbide and nitride as compared to the ordered structure of noncontacting octahedra. The interaction of the octahedral bonds in the corner-sharing structure of the carbide ( (E_{\mathrm{st}}=) (-4.9 \mathrm{mRy} / \mathrm{V}) atom) does not scem strong enough to predominate in forming carbide superstructures since vacancies avoid (f c c) secondneighbor positions. On the other hand, significantly larger stabilization in nitride (\left(E_{\mathrm{st}}=-13.2 \mathrm{mRy} / \mathrm{V}\right.) atom) agrees well with fec second-neighbor positions of vacancies in nitride superstructures. 1994 Acadenic Press, Inc.