An important byproduct of the IRI analysis is the IRI-π analysis, which refers to the IRI analysis that considers only π-electrons. This analysis can be conveniently performed with the help of our proposed algorithm for the automatic detection of π orbitals. In this analysis, electron density is mapped onto the IRI-π isosurface, typically with an isovalue of 1.0 a.u. For systems involving π-interactions, the IRI-π analysis can distinguish both the type and strength of these interactions. It is well known that the C-C bonds in ethylene and acetylene have one and two sets of πinteractions, respectively. As shown in Figs. ) and (b), their IRI-π images are quite distinct. The former shows two small, separated isosurfaces, while the latter displays a ring-shaped isosurface. Based on this difference, one can differentiate the characteristics of π-interactions in complex systems. Furthermore, by examining the magnitude of the electron density mapped onto the IRI-π isosurface, the strength of the same type of π-interactions can be compared. For example, a series of derivatives of cyclo [18]carbon, C18-(CO)n (n = 2, 4, 6), was recently investigated by us, the IRI-π map of C18-(CO)2 is shown in Fig. ). From the shape of the isosurfaces in this map, one can easily observe that there is only a single set of π-interactions in the CO addition region, while double sets of π-interactions (in-plane and out-of-plane) are present in other regions, which is similar to the situation in pristine cyclo [18]carbon. Additionally, as indicated by the purple arrows, the double sets of π-interactions on different C-C bonds exhibit notably different strengths. The relatively green and blue isosurfaces represent relatively low and high π-electron density in the corresponding regions, respectively, it is evident that the π-bond represented by the blue isosurface is significantly stronger than that represented by the green isosurface. reprinted with permission from Ref. . Copyright 2021 John Wiley and Sons. (c) is adapted from Ref. .