An ionic model that was originally built for chloroaluminate clusters by combining truncated expansions in classical multipoles and in quantal overlaps has been extensively applied to describe cohesion, structure, and vibrational spectra of a wide variety of molecular clusters in polyvalent metal halides [for a review see M.P. Tosi, Phys. Chem. Liq., 43, 409 (2005)]. In this work we test on simple ionic molecules two crucial aspects of the model, namely (i) the transferability of the overlap parameters for the halogen ions across families of halide compounds, and (ii) the anharmonicity in the halogen - metal ion interaction potential over a very broad range of interionic distances. Transferability is tested by means of a parallel discussion of alkali and alkaline-earth halide monomers near equilibrium. With regard to anharmonicity, the full potential energy curve yielded by the model for the sodium chloride monomer is compared with the results of quantum mechanical calculations for the ionic state of the molecule within a configuration-interaction approach.