Effects of chemical bonding and the details of the electron-density distribution of systems in condensed matter are at the heart of many applications in materials and life sciences, physics, chemistry, biology, mineralogy, and related fields. Therefore, quantum chemistry methods and crystal structure determination are indispensable research tools that have progressively been combined over many years for the determination of total electron-density distributions. Various techniques, methods and fields have resulted from this evolution, such as 3D electron diffraction, multipole modelling of X-ray diffraction data, periodic density functional theory, to only name a few.

The outcome of the first CECAM Discussion Meeting on Quantum Crystallography in 2017 (Nancy, 19-20 June) was to synergize all efforts concerning accurate and theoretical crystallography as well as experimental determination of charge and spin densities in position and momentum representations under the new umbrella term Quantum Crystallography (QCr). The participants summarized these thoughts into a new perspective of the field in ref. [1]. Consequently, within a year of the first CECAM meeting, the related Commission of the International Union of Crystallography and the Special Interest Group 2 of the European Crystallographic Association were re-established under the new name Quantum Crystallography (see their support letters attached to this application). This name goes back to early work by Massa, Huang, and Karle [2]. The background behind the original methods was reviewed in 2017 [3], but the field has already spread significantly since then, producing a recent special issue focussed entirely on QCr [4] and a future special issue in 2025, placing QCr in the framework of the UNESCO International Year of Quantum Science and Technology.

After a period of consolidation and growth from 2017, with many new methods and software programs having evolved in the meantime (see, e.g., the quantum box project initiated by this community, https://qubox.org), new challenges and opportunities have emerged:

• How can QCr and related fields mutually fertilize each other? This question has emerged as a crucial one during the ongoing webinar series “Distinguished Lectures on Quantum Crystallography and Complementary Fields” (https://qcrwebinar.chem.uw.edu.pl).
• As a data-driven field, how do we utilize artificial intelligence for QCr method development?
• How do we best harvest ideas of newcomers and junior researchers, while consolidating the foundations?
• How can we better unify and arrange technical and software developments?
• How can these technical developments be streamlined for scientific discovery?
• What are the future community-driven projects?

To tackle these questions, we plan to organize the workshop around young researchers and newcomers, including scientists from the periphery of QCr. We will ensure that it is these groups of scientists that lead the discussions, instead of established researchers. We believe that this format will be especially useful to develop ideas that make QCr benefit from the revolution of artificial intelligence and big-data treatment, for which the field is especially susceptible, while maintaining the school of thought and skill set unique to QCr.