The nature and importance of nonadditive three-body interactions in the ionic OH−(H2O)2 cluster have been studied by supermolecule Mo/ller–Plesset (MP) perturbation theory and coupled-cluster method, and by symmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Mo/ller–Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster calculations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze the three-body interactions in the clusters OH−(H2O)n, n=2,3,4,10, with water molecules located either in the first or the second solvation shell. It is shown that at the Hartree–Fock level the induction nonadditivity is dominant, but it is partly quenched by the Heitler–London and exchange-induction/deformation terms. This implies that the induction energy alone is not a reliable approximation to the Hartree–Fock nonadditive energy. At the correlated level, the most important contributions come from the induction-dispersion and the MP2 exchange energies. The exchange-dispersion and dispersion nonadditivities are much smaller, and for some geometries even negligible. This suggests that it will be difficult to approximate the three-body potential for OH−(H2O)2 by a simple analytical expression. The three-body energy represents only 4%–7% of the pair CCSD(T) intermolecular energy for the OH−(H2O)2 cluster, but can reach as much as 18% for OH−(H2O)4. Particular attention has been paid to the effect of the relaxation of the geometry of the subsystems.

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