Using fit functions in computational dielectric spectroscopy

This work deals with the development of an appropriate set of fit functions for describing dielectric spectra based on simulated raw data. All these fit functions are of exponential character with properly chosen cofunctions. The type of the cofunctions is different for translation, rotation and their coupling. As an alternative to multiexponential fits we also discuss Kohlrausch–Williams–Watts functions. Since the corresponding Fourier–Laplace series for these stretched exponentials has severe convergence problems, we represent their Fourier–Laplace spectrum as a Havriliak–Negami expression with properly chosen parameters. A general relation between the parameter of the Kohlrausch–Williams–Watts and the Havriliak–Negami parameters is given. The set of fit functions is applied to the concrete simulation of the hydrated ionic liquid 1-ethyl-3-methyl-imidazolium triflate with H2O. The systematic variation of the water mole fraction permits to study the gradual transition from a neutral molecular liquid to molecular ionic liquids.