Florian Dommert

Force Fields for Studying the Structure and Dynamics of Ionic Liquids: A Critical Review of Recent Developments

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.

Ionic Charge Reduction and Atomic Partial Charges from First-Principles Calculations of 1,3-Dimethylimidazolium Chloride

We present a detailed calculation of partial charges for the 1,3-dimethylimidazolium chloride ionic liquid. We first analyze MP2 electronic structure calculations and DFT results on isolated ion pairs with various methods of assigning partial charges to the atomic centers. In a second run we analyze the trajectory of a 25 ps long Car-Parrinello MD run of 30 ion pairs under bulk conditions using a charge fitting procedure due to Blöchl. Both, the single ion pair and the bulk system, provide us with a similar total ionic charge considerably less than unity. Especially the liquid state DFT results give convincing evidence for a reduced ionic charge on the ions. The similarity of both results suggest that the delocalization of the Cl charge is due only to local interactions. The relevance of our results is 2-fold; on the one hand they shed light on the basic property of the liquid and its reduced ionic character, and on the other hand, the ab initio derived partial charges provide a fundamental theoretical basis for the recent attempts to use the total ionic charge as an adjustable parameter. Furthermore, all our partial charges are subject to large fluctuations, hinting to the importance of polarization effects.

Optimizing working parameters of the smooth particle mesh Ewald algorithm in terms of accuracy and efficiency

We construct an accurate error estimate for the root mean square force error of the smooth particle mesh Ewald (SPME) algorithm, which is often used for molecular dynamics simulations, where charge configurations under periodic boundary conditions require considerable amounts of CPU time. The error estimates are provided for the ik- as well as analytical force differentiation schemes, and their validity is tested for a random homogeneous sample system. Finally, we demonstrate that it is possible to straightforwardly and precisely determine the SPME parameters via the error estimates prior to the simulation for a predetermined accuracy. This can save precious computer and user time and allows an easy choice of a suitable parameter set for nearly optimal speed.

Locality and Fluctuations: Trends in Imidazolium-Based Ionic Liquids and Beyond

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.