A Density-Functional Study of the Intermolecular Interactions of Benzene

We have tested the performance of three frequently used density functionals (LDA, LDA+B, and LDA+B+LYP) in a study of the intermolecular interactions of benzene. Molecular geometries are satisfactory with the gradient-corrected density functionals yielding slightly better results. The quadrupole moment is significantly underestimated by all three functionals. LDA performs fortuitously comparatively well for both binding energies and geometries of the dimer and the solid, whereas in LDA+B, and LDA+B+LYP the dimer interaction is purely repulsive, leading to the complete absence of cohesion in the solid. These results are consistent with density-functional theory calculations for noble gas dimers. However, when the dispersion energy calculated from a model potential is included, LDA fails. Binding energies are overestimated by unacceptable amounts, and intermolecular distances are too small. In constrast, dispersion corrected LDA+B and LDA+B+LYP perform reasonably well, although discrepancies are still large when measured on the thermal energy scale at room temperature.