intermolecular interactions

 Dear Net Chemists
 Could some Quantum chemistry experts please offer some advice or
 comments on the computation of intermolecular interactions by
 quantum methods.
 I noticed a recent paper in which the the difference in the (SCF)
 energy of an ion pair at some separation (r) relative to the the sum
 of the energies of the individual ions was used to estimate the stabilisation
 of the ion pair. Does this approach neglect basis set superposition effects
 where the 'effective' basis set size grows as the two separate molecules
 are brought together. In addition how would such calculations be affected
 by numerical basis sets (eg as used by DMOL).
   Dr Mark J Forster Ph.D.
   Principal Scientist
   Informatics Laboratory
   National Institute for Biological Standards and Control
   Blanche Lane, South Mimms,
   Hertfordshire EN6 3QG, United Kingdom.
   Tel 	01707 654753
   FAX 	01707 646730
   E-mail 	mforster