  The infrared absorption spectrum of the protonated water dimer (H5O2+) is
simulated in full dimensionality (15D) in the spectral range 0-4000 cm-1. The
calculations are performed using the Multiconfiguration Time-Dependent Hartree
(MCTDH) method for propagation of wavepackets. All the fundamentals and several
overtones of the vibrational motion are computed. The spectrum of H5O2+ is
shaped to a large extent by couplings of the proton-transfer motion to large
amplitude fluxional motions of the water molecules, water bending and
water-water stretch motions. These couplings are identified and discussed, and
the corresponding spectral lines assigned. The large couplings featured by
H5O2+ do not hinder, however, to describe the coupled vibrational motion by
well defined simple types of vibration (stretching, bending, etc.) based on
well defined modes of vibration, in terms of which the spectral lines are
assigned. Comparison of our results to recent experiments and calculations on
the system is given. The reported MCTDH IR-spectrum is in very good agreement
to the recently measured spectrum by Hammer et al. [JCP, 122, 244301, (2005)].
