First, different approaches to detect hydrogen bonds and to evaluate their energies are introduced newly or are extended. Supermolecular interaction energies of 256 dimers, each containing one single hydrogen bond, were correlated to various descriptors by a fit function depending both on the donor and acceptor atoms of the hydrogen bond. On the one hand, descriptors were orbital-based parameters as the two-center or three-center shared electron number, products of ionization potentials and shared electron numbers, and the natural bond orbital interaction energy. On the other hand, integral descriptors examined were the acceptor-proton distance, the hydrogen bond angle, and the IR frequency shift of the donor-proton stretching vibration. Whereas an interaction energy dependence on 1/r(3.8) was established, no correlation was found for the angle. Second, the fit functions are applied to hydrogen bonds in polypeptides, amino acid dimers, and water cluster, thus their reliability is demonstrated. Employing the fit functions to assign intramolecular hydrogen bond energies in polypeptides, several side chain CH...O and CH...N hydrogen bonds were detected on the fly. Also, the fit functions describe rather well intermolecular hydrogen bonds in amino acid dimers and the cooperativity of hydrogen bond energies in water clusters.

Classical molecular dynamics simulations are a valuable tool to study the mechanisms that dominate the properties of ionic liquids (ILs) on the atomistic and molecular level. However, the basis for any molecular dynamics simulation is an accurate force field describing the effective interactions between all atoms in the IL. Normally this is done by empirical potentials which can be partially derived from quantum mechanical calculations on simple subunits or have been fitted to experimental data. Unfortunately, the number of accurate classical non-polarizable models for ILs that allow a reasonable description of both dynamical and statical properties is still low. However, the strongly increasing computational power allows one to apply computationally more expensive methods, and even polarizable-force-field-based models on time and length scales long enough to ensure a proper sampling of the phase space. This review attempts to summarize recent achievements and methods in the development of classical force fields for ionic liquids. As this class of salts covers a large number of compounds, we focus our review on imidazolium-based ionic liquids, but show that the main conclusions are valid for non-imidazolium salts, too. Insight obtained from recent electronic density functional results into the parametrization of partial charges and on the influence of polarization effects in bulk ILs is highlighted. An overview is given of different available force fields, ranging from the atomistic to the coarse-grained level, covering implicit as well as explicit modeling of polarization. We show that the recently popular usage of the ion charge as fit parameter can looked upon as treating polarization effects in a mean-field matter.

For theoreticians, ionic liquids represent a major challenge. This is due to the fact that intermolecular interactions are particularly strong because of ionic liquids' ionicity. This, in turn, causes a subtle interplay between different scales which is encoded in the measured macro- and mesoscopic properties and also in the molecular electrostatic characteristics. Therefore, force fields have to describe the microscopic processes correctly in order to reproduce macroscopic properties accurately over a large range of state variables. Herein, imidazolium-based ionic liquids were studied at different scales, going from the detailed quantum electronic scale to the classical atomistic scale. It is indicated how the information gained at each level could be used for the other scales. In particular, the issue of deriving suitable partial charges for use in classical force fields is addressed. The Blöchl method was employed to generate partial charges reproducing the multipole distribution accurately for bulk systems. This led naturally to absolute ionic charges of less than /l e/, i.e., charge scaling. So, the monopole structure of the herein introduced force field mimics the quantum chemical behaviour observed in the liquid phase. This led to a substantial improvement in the description of dynamical properties of immediate experimental interest, such as electric conductivity. For further insight, the electric dipole moment of the ions was taken as physical indicator of their electronic structure. The electric dipole moment was found to fluctuate strongly and to depend on polarisation. Hence, our scale-combined study offers a gateway to rational design of models, based on the relevant underlying physics rather than on mere numerical parameterisation, and thereby to (possibly) more direct physical interpretation of experimental results.

Three different imidazolium-based ionic liquids, 1,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium thiocyanate, and 1-ethyl-3-methylimidazolium dicyanamide, are investigated by Car-Parrinello simulations. A common behavior, such as a broad electric dipole moment distribution of the ions and a related high degree of locality, is found to characterize all these systems. Going beyond imidazolium-based systems, we found that even for the protic ionic liquid monomethyl ammonium nitrate, the same features hold. These results represent a strong support to the hypothesis of rattling ions in long-living ion cages proposed in the last years.

Power spectra of several imidazolium-based ionic liquids, 1,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium dicyanamide 5, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium thiocyanate, and 1-butyl-3-methylimidazolium dicyanamide, are presented based on ab initio molecular dynamics simulations. They provide an alternative tool of analysis of several electronic structure-based properties, in particular, those related to the strength of hydrogen bonding in liquids. Moreover, they can be employed to interpret experimental IR or Raman spectra, avoiding the additional calculations required for theoretical IR or Raman spectra. The obtained power spectra are shown to be in good agreement with experimental spectra, and electronic structure properties related to them are analyzed. Further, there are indications for a locality of the power spectra on a relatively short time scale of ≈10 ps or system size of about 8 ion pairs as already speculated in previous work.