The interaction energies for the crystal of p-benzoquinone and aggregates from two to seven molecules have been calculated using Hartree-Fock and density functional theory methods with different basis sets at the experimental (crystallographic) geometries. After correction for basis set superposition error, all methods predict similar interaction energies. Periodic calculations using CRYSTAL92 and CRYSTAL 95 at the HF/ 6-21G** level produced values of -5.30 kcal/mol for the interaction energy of a two-dimensional slab. The calculation of the crystalline interaction energy is hampered by the inability of HF calculations to reproduce stacking interactions and the large basis set superposition errors associated with the HF/G-21G** molecular orbital method. The cooperative contributions to the interaction energies calculated for the aggregates imply that roughly half the crystalline interaction energy is due to cooperativity.