Nonrigidity effects in the He(1s2s)+H$_2$ collision pair

Low-energy collisions exhibit the quantum nature of matter and physical phenomena. In our work, we show a crucial role of the flexibility of molecule in anisotropic atom--diatom collisions in sub-kelvin regime. We study Penning ionization reactions between excited helium atoms and ground state hydrogen molecules. Our results from state-of-the-art {\it ab initio} calculations are in excellent agreement with the recent experimental findings [Klein {\it et al.} Nature Phys. 13, 35--38 (2017)]. We reveal that the nonrigidity effect of H$_2$ on the reaction rate structure, not recognized in the previous study, is indispensable to correctly describe the observed resonances without a need for any empirical adjustment. We demonstrate that the approach beyond the widely used rigid-rotor approximation is required even when rigorous computations are carried out at the FCI level of theory.