ECA lunchtime webinars - Quantum crystallography for studying dynamics and disorder in crystals

L'Association Européenne de Cristallographie (ECA) organise depuis 2021 des "webinaires déjeuner". Les groupes d'intérêt spéciaux (SIG) et les groupes d'intérêt général (GIG) de l'ECA organisent à tour de rôle un webinaire adapté à la communauté des cristallographes au sens large. Le 46ième webinaire sera diffusé le 12 novembre 2026 à 13h. Le thème de ce webinaire est Quantum crystallography for studying dynamics and disorder in crystals (Anders Madsen, University of Copenhagen, Denmark). Inscriptions ici.

Crystals are complicated. A thorough model of a crystal must describe not only the atomic positions, but also the atomic and molecular vibrations and possible disorder.
A plethora of important bulk solid state properties depend on crystal vibrational properties and disorder. The molecular motion and disorder affect the enthalpy and entropy of crystals and are therefore crucial for understanding stabilities and phase changes, e.g. the relative stabilities of polymorphs, hydrates and solvates that matter for many important classes of materials, including pharmaceuticals

In recent years, we have developed an approach to refine vibrational modes – derived from periodic ab-initio calculations – against X-ray diffraction data. This approach, called normal mode refinement (NoMoRe), have been used in studies of phase transformations and polymorphic stabilities in molecular crystals.
We have combined NoMoRe with aspherical atom models such as Hirshfeld atom refinement or the transferable aspherical atom model, in a single refinement, which gave heat capacities in good agreement with calorimetry.

An interesting development is to replace the DFT calculations with machine-learned interatomic potentials (MLIPs). This substantially lowers the cost of the phonon calculations, making NoMoRe refinements applicable in a standard crystallographic refinement workflow.

MLIPs also open the door for a more thorough examination of the possibilities and limitations of the NoMoRe approach, because MLIPs allow fast calculation of ADPs over a full Brillouin-zone sampling, providing a theoretical ground truth in the harmonic approximation.
We illustrate the NoMoRe approach with the examination of relevant polymorphs of pharmaceutical interest, and show how NoMoRe in combination with analysis of inelastic X-ray scattering, diffuse scattering and low-frequency Raman data can give insights into the dynamics associated with the phase transition in ‘jumping crystals’.
Finally we introduce the NoMoRe web server which makes the method available to other users, with an integrated thermodynamics module.