Density-functional Calculation of Anisotropic Displacement Parameters and its Use for Improving Experimental X-ray and Neutron Diffraction

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Because of new approaches covering dispersion interactions, DFT methods for modelling molecular crystals have significantly gained attraction. Our proposal goes beyond structural optimization of molecular solids, however: we intend to use DFT+D methods for phonon calculations on van-der-Waals dominated solids to arrive at first-principles displacement parameters. By doing so, we will evaluate which kind of dispersion correction is particularly suited for such question and which temperature range is appropriate for the (primarily) harmonic approximation. Likewise, potential restrictions as regards elemental composition, intermolecular interactions and cell size of the model systems will be investigated. Parallel to our theoretical studies, we will synthesize suitable molecular crystals and characterize them using diffraction in a large temperature range below room temperature but with the highest possible resolution. This very cooperation between theoretically and experimentally inclined structural chemists will guarantee, on the one hand, the reliable validation of even more advanced theoretical approaches in the making and, on the other hand, lead to an improved handling of experimental diffraction data. The first aspect will allow us to rudimentally include anharmonic motion based on experimentally derived expansion coefficients and, thus, to increase our method's scope. The second aspect alludes to unfolding the electron density and the thermal motion which is particularly helpful for crystal structures showing problems of contrast and correlation. We also expect certain advantages for the very ambitious question of experimental charge densities from highly resolved diffractional data: population parameters of atom-centered multipoles and displacement parameters are often strongly correlated; the use of calculated displacement parameters will allow for a more meaningful determination of these multipoles. Eventually, we would love to broadly establish our method such that it is perceived (and, hopefully, used) by other structural chemists. Hence, we plan to provide a self-explanatory web platform in addition to publications in peer-reviewed journals.