Quantum mechanical methods have been applied to the cis-ONOO--H2O, cis-ONOO--(H2O)2 and trans- ONOO--H2O complexes. Equilibrium geometries, binding energies, net atomic charges and vibrational frequencies are presented for several different arrangements. The MØller-Plessett second-order perturbation (MP2) method predicted shorter hydrogen bonds than the SCF method, but the computed Hartree-Fock (HF) binding energies are similar to counterpoise corrected MP2 values. The geometry changes of ONOO- and water after solvation are examined. The ONOO- and H2O bond length changes follow typical hydrogen bond structural trends, whereas bond angles in ONOO- are unaffected when the hydrogen bond is formed, similar to the conclusions from NO2--(H2O)n HF/6-31G studies and Monte Carlo simulations. The cis-ONOO--(H2O)n frequencies are compared with the solution Raman spectrum and with calculations on isolated ONOO-. © 1995 Plenum Publishing Corporation.