Inversion barriers and geometries of AB3 molecules calculated using the INDO method3 are in satisfactory agreement with experiment. The diazacumulenes (11) with an even number of atoms are calculated to have a higher configurational stability than those with an odd number of atoms. cis-Diimide and cis-difluorodiimide should be more stable thermodynamically than the corresponding trans isomers. The configurational stability of carbodiimide is predicted to be low (comparable to that of ammonia), the isomerization being a combination of rotation and inversion, However, difluorocarbodiimide should be stable enough to permit resolution.