In this work we test two ab initio methodologies which allow the decomposition of the total intermolecular interaction energy into physically meaningful contributions, namely, the symmetry adapted perturbation theory (SAPT) and the use of localized orbitals within a Moller-Plesset perturbation scheme. The accuracy of the two different methods is compared to supermolecular results, within MP2 and coupled-cluster theory within single and double excitations, with perturbative estimates of the amplitudes of triple excitations [CCSD(T)]. Some relations between the different approaches are conjectured from theoretical considerations, and are confirmed by numerical results. The corresponding calculations have been performed for three model dimers: two NH3.H2O dimers, with NH3 acting once as a proton acceptor and once as a proton donor, and the NH4+.H2O considered as a prototype of the ion-molecule interaction. We may conclude that third-order terms in SAPT help significantly to reproduce the Hartree-Fock induction and the relaxed, total dispersion in the LMP2 decomposition. (C) 2003 American Institute of Physics.