The role of charge resonances in the benzene dimer

TL;DR

The paper addresses whether dispersion alone captures the benzene dimer's potential energy surface or whether charge resonances via ion-pair configurations are also essential. It uses a localized Hartree–Fock CCSD framework and a charge-localized variant (cl-CCSD) that omits ion-pair amplitudes to isolate dispersion from charge-delocalization effects, with BSSE-corrected energies. The results show that ion-pair configurations markedly alter the PES, shifting the parallel-displacement minimum by about 2 Å and reducing short-range binding energy by roughly a third to a half, while long-range behavior remains dispersion-dominated. The study demonstrates that both dispersion and charge resonances govern the benzene dimer's noncovalent bonding, and it highlights the importance of properly accounting for BSSE when diagnosing charge-delocalization contributions.

Abstract

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 non-covalent bonding in the benzene dimer two mechanisms must be taken into account: dispersion interaction and charge resonances.

Figures (3)

• Figure 1: An illustration of connected doubles excitations in $T_\mathcal{AB}$. a: excitations which are in $\hat{T}_\mathcal{AB}^\text{neutral}$. The two processes represent dispersion. b: Processes which generate singly ionic configurations and which are in $\hat{T}_\mathcal{AB}^\text{ionic}$. c: Processes which generate doubly ionic configurations and are also in $\hat{T}_\mathcal{AB}^\text{ionic}$.
• Figure 2: The interaction energy computed using counterpoise corrected CCSD and cl-CCSD as a function of horizontal displacement, $d$, on an interval symmetric around the sandwiched ($d=0$) geometry. The calculations use aug-cc-pVDZ on the carbon atoms and cc-pVDZ on the hydrogen atoms. The coordinate is illustrated by the dimer geometries shown above the plot, made using UCSF Chimera.Pettersen:2004aa
• Figure 3: CCSD and cl-CCSD counterpoise corrected interaction energies as a function of intermolecular separation, $R$. The displacement coordinate is kept fixed at $d=1.6$ Å. The calculations use aug-cc-pVDZ on the carbon atoms and cc-pVDZ on the hydrogen atoms.