The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and secondâ€�order Møller–Plesset levels, using the 6â€�311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogenâ€�bonded O...O...O (O₃) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O₃ plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrationalâ€�tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C₆ symmetry and a very low barrier V₆=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further lowâ€�energy saddle points on the surface are of second and third order; they correspond to crownâ€�type and planar geometries with C₃ and C_3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetryâ€�adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed.