Dispersion energy of a van der Waals complex of an aromatic molecule and a rare-gas atom

The dispersion energy of a van der Waals complex of a rare-gas atom and an aromatic hydrocarbon is shown to have two components with radically different properties. That part which involves the σ electrons in the aromatic has conventional form and can be expressed as the sum of atom–atom terms, whereas that part involving the π electrons is the sum of three-atom terms relating the rare-gas atom and pairs of carbon atoms in the aromatic. Asymptotically both parts have the expected R–6 dependence on separation distance, R, but at distances comparable with the equilibrium separation distance the behaviour is much more complicated. Numerical results for a number of important cases are presented and used to examine how this more theoretically satisfactory approach differs from the widely used method based on empirical, atom–atom potentials.