Browsing by Author "Dommert, Florian"

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Open Access
From the inhomogeneous electron gas to classical force fields : a multi-scale model for ionic liquids
(2013) Dommert, Florian; Holm, Christian (Prof. Dr.)
Ionic liquids are a class of solvents that have attracted a broad interest in the recent decade. They are often liquid at room temperature, but consist only of cations and anions without any additional solvent. The characteristic properties of these liquids are the negligible volatility and their tunability. Thus they are often referred to as designer solvents, which can be adapted in order to increase the efficiency of many applications like catalysis, capturing of flue gases, or solar cells. The number of different ionic liquids is quite large and an experimental screening of all ionic liquids would be very expensive and time-consuming. At this stage, classical molecular dynamics simulations are a very suitable tool that allows one to study thermodynamic, structural, and dynamic properties of liquids, but they depend on an accurate force field. For ionic liquids, reliable and transferable force fields are rare. For this reason, the main object of this work is to establish a method to optimize or generate a set of force field parameters. This is achieved by mapping the electronic and geometric structure information of the liquid phase gathered on the DFT level to the classical scale. Apart from our test system dimethylimidazolium chloride [MMIM][Cl] , which was used to develop corresponding methods, we investigated a broad spectrum of imidazolium-based cations combined with the anions thiocyanate [SCN]-, dicyanamide [DCA]-, and chloride [Cl]-. These studies were based on CPMD simulation snapshots, which allowed one to gain insight into the electronic structure of ionic liquids, especially in mechanisms of partial charge distribution and the net—charge reduction, which has been identified as a model for implicit polarization and charge transfer. These features are not only characteristic for an IL, but it has also been shown, that they occur already on a very local scale, which is the reason for the good performance of partial charges that were derived from small ion pair clusters. It was shown that the predicted charge transfer and polarization is in agreement with NMR experiments and measurements of the refractive index. From the computational side, consistency of the dipole moments given by the partial charges or by a Wannier analysis of the CPMD results was achieved. Though the width of dipole moment distribution could not be completely reproduced by the static partial charges, the increase in the dipole moment for an increasing number of interacting ion pairs (IPs) coincides very well. This shows that the proposed set of partial charges is perfectly suitable for the bulk phase of an IL. Finally the short—range (SR) interactions were adapted in respect to our proposed set of partial charges. An algorithm was implemented that optimizes the SR parameters based on an iterative adaption to match static properties given by the experimental mass density and the radial distribution functions derived from CPMD simulations. The routine has been successfully tested for [MMIM][Cl] and it has been shown that the implicit consideration of polarization and charge transfer models the dynamics already well, such that one can rely on static properties during the SR parameter optimization. This alleviates the tuning process, because the amount of simulation time required to sample a highly viscous IL decreases and, apart from the experimental mass density, only computational results are required. The latter aspect is of great importance for ILs, because a broad spectrum of experimental knowledge is not a given. Thus, during this work, the technique and computational framework to optimize FF parameters for ionic liquids was established and a large data set was accumulated. Thus, a solid basis for future work is provided, the development of a generic and transferable force field for ILs.