Ions of molecular complexes with internal interactions partly of van der
Waals type differ significantly from their neutral parent species in binding, structure, and dynamics. Information about such systems is still rather scarce, both experimentally and theoretically. The article discusses some aspects of the present knowledge concerning cationic dimers and trimers of the type Rg M q with Rg denoting a rare-gas atom, and M, n Ž . either a diatomic molecule or a rare-gas atom, in comparison with the neutral Ž . counterparts. In some more detail, potential energy surfaces PESs for the six lowest electronic states of ArHCl q are reported, as obtained from multireference CI calculations with an extended Gaussian AO basis set. From the PESs, structural, energetical, and dynamical features are derived. For the cationic argon trimer Ar q in the electronic 3 ground state, a detailed analysis of the PES, the mode couplings, and the classical internal dynamics is carried out, leading to a consistent picture of a floppy molecular aggregate with rapid energy exchange between internal degrees of freedom and early onset of classical chaos with increasing energy.