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

The complex interplay of long-range van der Waals (vdW) attraction and shortrange (Pauli) repulsion plays a major role for the structure and dynamics of molecular systems. Arising from different physical origins, these forces do not seem to have a simple connection. Here, we build a consistent approach to study these interactions within the coarse-grained quantum Drude oscillator (QDO) model. This is an efficient tool to describe long-range correlation forces, where response properties of all valence electrons are represented just by a Drude particle. Treating these Drude oscillator particles as indistinguishable, we introduce the Pauli repulsion into this model and evaluate the exchange energy based on the multipole expansion of the Coulomb interaction. This allows us to reveal a remarkable compensation between dispersion and exchange forces for closed-shell dimers. The hidden symmetry between these two energies explains the recently discovered relationship between the vdW radius and atomic polarizabilities with a surprising connection between electronic and geometric properties of quantum-mechanical systems. Altogether, our results pave the way to explore the potential usefulness of the QDO model as a complete building block to develop next-generation quantum force fields.