Coordinate systems, PES representation and angular cones. Top left: coordinate system used for the description of the He^+⋅ + CH₃CN interaction. Due to the cylindrical symmetry of CH₃CN and CH₃ CN^+⋅ (in our formulation, where the CH₃ group is treated as a single effective atom), the PES (as for an atom-linear molecule system) depends solely on the polar coordinates R and θ. Top right: 3D representation of the PES, in the xz (or yz) plane, in the entrance channel. The attractive and repulsive contributions are indicated in blue and yellow, respectively. Bottom: representation of the three different angular cones cone_i (with i = 0, 1, 2) used in the reaction dynamics treatment discussed in Sect. 4.4.