Beyond Candidate Generation: A Validation-First Protocol for AI-Assisted Transition-State Discovery

Machine-learned models can now propose three-dimensional transition-state (TS) geometries in seconds, but a candidate geometry is not a validated TS. The gap between a plausible structure and a mechanistically meaningful barrier—one imaginary frequency, the intended atom pair, the correct connectivity, and a consistent electronic state—remains the bottleneck in AI-assisted computational chemistry. We present a validation-first protocol that separates candidate generation from physical validation. The protocol combines (i) a chemical preflight that fixes reactive pairs, spin surfaces, connectivity, and conformers; (ii) a per-point independent scan that localizes the barrier without fragile collective scans; (iii) a constrained reference calculation that produces the Hessian; (iv) an OptTS stage that reuses that Hessian; and (v) a validation stage based on imaginary-mode localization, key bond lengths, IRC endpoints, spin diagnostics, and energy-convention checks. The protocol is implemented as a portable, open-source skill package for local ORCA/xtb and Gaussian/Slurm execution. Case studies from Ni/Fe catalysis illustrate the failure modes the protocol is designed to catch: a generative model produced candidates with 3.6–4.4 Å heavy-atom RMSD for an 84-atom broken-symmetry organometallic system, a collective scan failed three times where a per-point strategy completed 16/16 points, and a single-conformer R/S comparison would have misread a conformerdominated difference. The reported gains of multi-sampling and conformer enumeration are taken from the published benchmark, not measured here. We argue that the near-term role of AI is not to replace electronic-structure judgment but to shrink the search space before it and to make the validation layer reproducible.