  The dinitrides CN2, SiN2, and GeN2 in assumed pyrite-type structures are
studied by means of density functional theory using both ultrasoft
pseudopotentials and the augmented spherical wave (ASW) method. The former two
materials constitute the large-x limit of the broader class of CNx and SiNx
compounds, which are well known for their interesting mechanical and electronic
properties. For CN2 a large bulk modulus B_0 of 405 GPa was determined . While
SiN2 is found to be a wide band gap compound, the calculated gaps of CN2 and
GeN2 are considerably smaller. The trends in structural and electronic
properties as e.g. bond lengths, band gaps and covalency are well understood in
terms of the interplay of different types of bonding.
