Van der Waals Attraction and Pauli Repulsion: Learning New Tricks from an Old Dog

The structure and stability of molecular systems bonded by van der Waals (vdW) interactions are governed by the interplay between dispersion attraction and Pauli repulsion [Hermann et al., Chem. Rev. 117, 4714 (2017)]. Arising due to different physical origins, these forces do not seem to have a simple connection. Here, we present a coarse-grained approach for evaluating the exchange energy based on the multipole expansion of the Coulomb interaction. This allows us to reveal an unexpected compensation between attractive dispersion and repulsive exchange forces for closed-shell dimers at equilibrium distance, valid for each term of the multipole expansion. This effect explains the surprisingly simple relationship between the vdW radius and atomic polarizabilities discovered recently [Fedorov et al., PRL 121, 183401 (2018)]. The obtained recipe to build coarse-grained models for the exchange-repulsive forces in vdW-bonded systems could be used to develop next-generation quantum force fields. Therefore, our findings hint on a hidden symmetry between exchange and correlation interactions and suggest a surprising connection between electronic and geometric properties of atoms.