First-principles quantum mechanical calculations have been widely successful in describing chemical processes at a fundamental level. However, the interpretability of these calculations is still an ongoing subject of debate . How does one move between the electrons and nuclei of first-principles calculations to the more intuitive building blocks of chemistry, such as atoms, bonds, lone-pairs etc.? Significant progress has been made in the forward direction by considering the topography and topology of quantum-mechanical objects, in a field that has become known as quantum-chemical topology (QCT) . Early examples, such as Bader's partitioning of atoms-in-molecules (AIM) via the basins of attraction of the electron density ρ, demonstrated that it was possible to recover the concepts of atoms and bonds . Later, such methods were generalized and applied to properties such as the electron localization function (ELF) and the Laplacian of the density, ∇ 2 ρ , which elucidate the role and locations of lone pairs and core and valence regions in chemical reactions. The Laplacian can also be used to delimit regions of strong and weak correlation and is crucial to the construction of kinetic energy functionals . The relationship between topology and the description of the overall system as a set of open quantum sub-systems, as initially demonstrated by Bader , has also been generalized .