# Nonrigidity effects -- a missing puzzle piece in the description of low-energy anisotropic molecular collisions

## TL;DR

This study demonstrates that accounting for the nonrigidity of H₂ molecules is essential for accurately predicting resonance positions and reaction rates in low-energy anisotropic molecular collisions, significantly improving theoretical-experimental agreement.

## Contribution

The paper introduces the inclusion of molecular nonrigidity into interaction potential models, revealing its critical role in accurately describing cold molecular collision resonances.

## Key findings

- Nonrigidity effects are crucial for correct resonance predictions.
- Inclusion of molecular flexibility improves agreement with experimental reaction rates.
- Neglecting nonrigidity leads to significant discrepancies in cold collision modeling.

## Abstract

Cold collisions serve as a very sensitive probe of the interaction potential. In the recent study of Klein et al. (Nature Phys. 13, 35-38 (2017)) the one-parameter scaling of the interaction potential was necessary to obtain agreement between theoretical and observed patterns of the orbiting resonances for excited metastable helium atoms colliding with hydrogen molecules. Here we show that the effect of nonrigidity of the H$_2$ molecule on the resonant structure, absent in the previous study, is critical to predict correct positions of the resonances in that case. We have complemented the theoretical description of the interaction potential and revised reaction rate coefficients by proper inclusion of the flexibility of the molecule. The calculated reaction rate coefficients are in remarkable agreement with the experimental data without empirical adjustment of the interaction potential. We have shown that even state-of-the-art calculations of the interaction energy cannot ensure agreement with the experiment if such an important physical effect as flexibility of the interacting molecule is neglected. Our findings about the significance of the nonrigidity effects can be especially crucial in cold chemistry, where the quantum nature of molecules is pronounced.