  The applicability of the density functional based tight binding (DFTB) method
to the description of hydrogen bond dynamics and infrared spectroscopy is
addressed for the exemplary protic ionic liquid triethylammonium nitrate.
Potential energy curves for proton transfer in gas and liquid phase are shown
to be comparable to high level coupled cluster theory in the thermally
accessible range of bond lengths. Geometric correlations in the hydrogen bond
dynamics are analyzed for a cluster of six ion pairs. Comparing DFTB and
regular DFT data lends further support for the reliability of the DFTB method.
Therefore, DFTB bulk simulations are performed to quantify the extent of
geometric correlations in terms of Pauling's bond order model. Further,
infrared (IR) absorption spectra are obtained and analyzed putting emphasis on
the signatures of hydrogen bonding in the NH-stretching and far IR hydrogen
bond range.
