Molecular orbital study of crystalline p-benzoquinone

The interaction energies for the crystal of p-benzoquinone and aggregates f rom two to seven molecules have been calculated using Hartree-Fock and dens ity functional theory methods with different basis sets at the experimental (crystallographic) geometries. After correction for basis set superpositio n error, all methods predict similar interaction energies. Periodic calcula tions using CRYSTAL92 and CRYSTAL 95 at the HF/ 6-21G** level produced valu es 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 in ability of HF calculations to reproduce stacking interactions and the large basis set superposition errors associated with the HF/G-21G** molecular or bital method. The cooperative contributions to the interaction energies cal culated for the aggregates imply that roughly half the crystalline interact ion energy is due to cooperativity.