Learning the Committor Function using Weighted Ensemble Simulations

Understanding how complex molecular systems transition between metastable states is a central challenge in computational chemistry and biophysics. A key tool for characterizing these transitions is the committor function, defined as the probability that a trajectory initiated from a given configuration reaches one state before another, which encodes detailed mechanistic information about transition pathways. First, we curate a public benchmark of empirically determined committor values for alanine dipeptide. Second, we develop a "committor-in-the-loop" framework that combines a self consistent loss function with weighted ensemble (WE) simulation: WE adaptive bins are biased by the learned committor, which is in turn trained on WE samples. Across a two-channel 2D system and alanine dipeptide settings, our WE-based framework consistently outperforms greedy baselines, demonstrating the importance of principled exploration. Together, the benchmark and framework provide both a standard for evaluating committor learning methods and a new method for studying rare transitions in high-dimensional molecular systems.