  Comparative molecular dynamics simulations of a hexamer cluster of the protic
ionic liquid ethylammonium nitrate are performed using density functional
theory (DFT) and density functional-based tight binding (DFTB) methods. The
focus is on assessing the performance of the DFTB approach to describe the
dynamics and infrared spectroscopic signatures of hydrogen bonding between the
ions. Average geometries and geometric correlations are found to be rather
similar. The same holds true for the far-infrared spectral region. Differences
are more pronounced for the NH- and CH-stretching band, where DFTB predicts a
broader intensity distribution. DFTB completely fails to describe the
fingerprint range shaped by nitrate anion vibrations. Finally, charge
fluctuations within the H-bonds are characterized yielding moderate
dependencies on geometry. On the basis of these results, DFTB is recommend for
the simulation of H-bond properties of this type of ionic liquids.
