van der Waals interactions of the benzene dimer: towards treatment of polycyclic aromatic hydrocarbon dimers

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Although density functional theory (DFT) in principle includes even long-range interactions, standard implementations employ local or semi-local approximations of the interaction energy and fail at describing the van der Waals interactions. We show how to modify a recent density functional that includes van der Waals interactions in planar systems [Phys. Rev. Lett. 91, 126402 (2003)] to also give an approximate interaction description of planar molecules. As a test case we use this modified functional to calculate the binding distance and energy for benzene dimers, with the perspective of treating also larger, flat molecules, such as the polycyclic aromatic hydrocarbons (PAH).