2D contour plot of the C₄H–He 2D-PES as a function of the intermolecular distance (R) and the angular coordinate (θ). The energy values, expressed in inverse centimeters, are referenced with respect to the energy of the C₄H + He system at the asymptotic dissociation limit. Negative energies are represented by blue contours with energy steps of 1 cm⁻¹, while the positive energies are depicted using yellow and red contours with energy steps of 200 cm⁻¹.