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

由于涵盖色散相互作用的新方法，DFT方法在分子晶体建模方面获得了显著的吸引力。然而，我们的建议超越了分子固体的结构优化：我们打算使用DFT+D方法对范德华主导的固体进行声子计算，以获得第一性原理位移参数。通过这样做，我们将评估哪种色散校正特别适合于这种问题，以及哪种温度范围适合于（主要）谐波近似。同样，将研究模型系统的元素组成、分子间相互作用和细胞大小方面的潜在限制。与我们的理论研究平行，我们将合成合适的分子晶体，并在室温以下的大温度范围内使用衍射来表征它们，但具有尽可能高的分辨率。这种理论和实验倾向的结构化学家之间的合作将保证，一方面，更先进的理论方法的可靠验证，另一方面，导致实验衍射数据的改进处理。第一个方面将使我们能够基本地包括基于实验推导的膨胀系数的非调和运动，从而扩大我们方法的范围。第二个方面涉及展开电子密度和热运动，这对显示对比和相关问题的晶体结构特别有帮助。我们还期望从高分辨衍射数据中得到的实验电荷密度问题具有一定的优势：原子中心多极的居群参数和位移参数通常是强相关的；使用计算的位移参数可以更有意义地确定这些多极。最终，我们希望能够广泛地建立我们的方法，以便其他结构化学家能够理解（并希望能够使用）。因此，除了在同行评议的期刊上发表文章外，我们计划提供一个自我解释的网络平台。

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.