TREK: Benchmarking Reaction-Level Transition-State Ensemble Recovery

Accurate transition-state generation for a known reactant–product pair does not establish recovery of the alternatives associated with a reaction. Measuring this broader capability requires a benchmark whose unit is the reaction and whose output is a set. We introduce **TREK** (Transition-state Recovery for Ensemble Kinetics), a benchmark for budgeted TS-ensemble recovery from complete retained reactant and product conformer sets. TREK organizes 3,493 reactions, 10,865 candidate endpoint pairs, and 19,647 xTB-derived proxy channels into two controlled tasks: known-pair one-shot generation and reaction-level recovery with target-blind pair selection and a shared proposal budget. Pair-aware one-to-one scoring measures distinct reference recovery, while a stagewise decomposition attributes the recovery gap to selection, capacity, and geometry/diversity. Across five generators, leading models achieve 94.0–95.6% pair-hit rates on held-out reactions at 0.50 Å, alongside 74.3–76.1% complete-reference-set recovery in the ensemble task. TREK further identifies a tolerance-dependent shift from geometric precision to output capacity, quantifies the coverage gained per additional proposal, and isolates 38 shared zero-hit reactions enriched in internal endpoint rearrangement. Frozen DFT and bidirectional IRC audits provide complementary evidence for individual reference candidates and conformationally distinct connections. TREK establishes a common experimental basis for developing generators that recover reaction-level structural diversity, connecting accurate individual proposals to budget-efficient ensemble coverage.