Towards Quantum Advantage in Nonadiabatic Chemistry: A Rigorous Framework for Encodings, Error Budgets, and Benchmarking

Quantum advantage claims in chemical dynamics are difficult to evaluate when critical assumptions---model reductions, discretizations, Hamiltonian access models, numerical precision, and validation practices---are implicit.Focusing on discretized diabatic nonadiabatic Hamiltonians $H_{\mathrm{dia}}({Q}) = T_Q\otimes \mathbb{I}_N + V_{\mathrm{dia}}({Q})$, we propose a unified theoretical and methodological framework that standardizes:(i) qubit register encodings and physical-subspace embeddings including a leakage diagnostic when dimensions are not powers of two;(ii) Hamiltonian access models, i.e., explicit decompositions versus sparse-oracle access with fixed-point value semantics;(iii) instance-parameterized resource envelopes for Hamiltonian simulation and observable estimation;(iv) an additive error-budget ledger separating modeling/truncation error from oracle precision, unitary simulation error, estimation error, leakage, and hardware noise; and(v) a benchmark problem family $\mathcal{P}_{\mathrm{nonad\,chem}}$ together with prototype instance ladders and a reporting checklist for matched classical--quantum comparisons.To support reproducibility without requiring software dependencies, we further specify a lightweight intermediate representation (IR) as a proposed data standard for nonadiabatic benchmark instances, including units, conventions, discretizations, embeddings, access declarations, and precision metadata.This paper is intentionally free of numerical results and implementation claims; it is a proposed baseline intended to make future algorithmic and experimental studies comparable and reviewable.