We have examined the basis set requirements to describe the properties of a prototype ionic molecule, LiCl. We relate, for the first time, the basis set requirements for this molecule to the demanding requirements for the accurate description of the dipole polarizability of the Cl anion. However, we are able to demonstrate that the basis set requirements in the molecule are ameliorated by a positive kind of basis set superposition effect from the basis functions on the counterion. (C) 2008 Elsevier B.V. All rights reserved.