The Role of Charge Resonances in the Benzene Dimer.

Modern electronic-structure theory defines dispersion interactions as connected intramonomer excitations. Using this definition, dispersion contributions have been shown in literature to be large relative to other contributions at van der Waals distances for the ground state benzene dimer. However, are the dispersion contributions sufficient to describe its potential energy surface? In this paper, we show the importance of charge resonances for the shape of the potential energy surface of the stacked benzene dimer. Charge resonances is a colloquial term for the presence of ion-pair configurations in the electronic wave function, and they represent a charge delocalization between the benzene molecules. We show that the ion-pair configurations, generated from connected intra and intermonomer excitations, have a significant impact on the potential energy curves as functions of parallel displacement, as well as intramonomer separation. For parallel displacement, the energy minimum shifts approximately 2 Å toward greater displacement if ion-pair configurations are not included. Hence, to understand the noncovalent bonding in the benzene dimer two mechanisms must be taken into account: dispersion interaction and charge resonances.