# Properties of Hydrogen Bonds in the Protic Ionic Liquid Ethylammonium Nitrate. DFT versus DFTB Molecular Dynamics

## TL;DR

This study compares DFT and DFTB molecular dynamics simulations of ethylammonium nitrate, evaluating their accuracy in describing hydrogen bonds and infrared spectra, and finds DFTB suitable for such analyses.

## Contribution

The paper assesses the performance of DFTB versus DFT in simulating hydrogen bonds in a protic ionic liquid, highlighting DFTB's strengths and limitations.

## Key findings

- DFTB predicts similar average geometries to DFT.
- Differences are notable in NH- and CH-stretching spectral regions.
- DFTB fails to accurately describe nitrate vibrational fingerprints.

## Abstract

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.