Nitrogen molecules N-x have been extensively studied for their potential as high energy density materials (HEDM). Cages of three-coordinate nitrogen have been studied to determine the structural features that result in the most stable isomers. N-12 cages, for example, have been shown to follow a general trend that molecules with more pentagons in the network are more stable than other molecules. Larger N-x cages (24 or more atoms) do not follow this trend, favoring cylindrical structures over spherical ones with larger numbers of pentagons. To determine which trends intermediate-sized N-x cages follow, a series of N-18 cages are studied by theoretical calculations of their stability. Geometries are optimized by using Hartree-Fock theory and perturbation theory (MP3), with single energies calculated with the fourth-order perturbation theory (MP4). The correlation-consistent CC-PVDZ basis of Dunning is employed. The major result is a loose trend favoring pentagons, with the most stable molecules being the ones with an optimal combination of pentagons and triangles.