Shown above is the geometry-optimized structure of the H5O2+ ion from ab initio MP2(aug-pVTZ) computations (L. Ojamäe, I. Shavitt and S. J. Singer, Int. J. Quantum Chem.: Quantum Chem. Symposium 29, 657 (1995)). For an animation of the vibrations of H5O2+, click here (260 kb). The amplitudes are given by the quantum-mechanical zero-point vibrations at 298 K.