The intermolecular interactions leading to crystals of m- and p-nitroaniline have been studied to better understand the interactions that lead to significantly differing crystal structures. Molecular orbital calculations using three semiempirical (AM1, PM3, and SAM1) Hamiltonians are presented for many aggregates containing up to 10 molecules of both m- and p-nitroaniline. The differences in the capacities for hydrogen bonding, both N-H ... O and C-H ... O, caused by the differing geometries of the monomeric units dictate the different crystal structures. p-Nitroaniline forms H-bonded chains with three-centered H-bonds that allow identical H-bonding on each side to adjacent chains. Thus, they form layers, which then stack (head-to-tail) to form centrosymmetric microcrystals. m-Nitroaniline, on the other hand, forms chains which can form much stronger H-bonds on one side than on the other. Thus, these chains combine in pairs to form "strands" comprised of two chains. These chains then stack in a head-to-head manner. The resulting "stacks" of strands then combine through C-H ... O H-bonds to others that are approximately perpendicular (104 degrees). While the head-to-head orientation of the stacks is somewhat unfavorable, it favors the combination of the stacks by properly positioning the C-H ... O interactions. Thus, a repulsive interaction is tolerated to allow for a more attractive one.