Deep learning of committor for ion dissociation and interpretable analysis of solvent effects using atom-centered symmetry functions

The association and dissociation of ion pairs in water are fundamental to physical chemistry, yet their reaction coordinates are complex, involving not only interionic distance but also solvent-mediated hydration structures. These processes are often represented by free-energy landscapes constructed from collective variables (CVs), such as interionic distance and water-bridging structures; however, it remains uncertain whether such representations reliably capture the transition pathways between the associated and dissociated states. In this study, we employ deep learning to identify reaction coordinates for NaCl ion pair association and dissociation in water, using the committor as a quantitative measure of progress along the transition pathway through the transition state. The solvent environment surrounding the ions is encoded through descriptors based on atom-centered symmetry functions (ACSFs), which serve as input variables for the neural network. In addition, SHapley Additive exPlanations analysis, as an explainable artificial intelligence technique, is applied to identify ACSFs that contribute to the reaction coordinate. A comparative analysis of their correlation with CVs representing water-bridging structures, such as interionic water density and the number of water molecules coordinating both ions, further provides a molecular-level interpretation of the ion association-dissociation mechanism in water.