Analysis of ab initio wave functions shows that the interaction between halogen atoms (F, Cl, and Br) and Ag clusters is ionic, and that the halogen ionicity is essentially -1. The interaction and the bonding arise, almost entirely, from the Coulomb attraction between the charged halogen and the metal and from polarization of the two subunits. Large shifts in equilibrium bond distances and vibrational energies are caused by an external electric field. These changes arise from a dominant Stark effect. © 1989 Springer-Verlag.