Spin–Orbit-Induced Nonadiabatic Dynamics: An Exact Ω Representation

Transforming rovibronic Hamiltonians of molecular systems from the ΛS (Hund’s case a) basis to the adiabatic Ω representation is widely used to “remove” spin–orbit coupling (SOC) and enable single-state treatments of spectra and dynamics. We show that this simplification is only apparent: the SOC elimination necessarily generates sizable nonadiabatic couplings (NACs) from the nuclear kinetic energy operator. Neglecting these spin–orbit-induced NACs causes severe errors in rovibronic energies and transition properties. Using an analytically tractable two electronic-state model and high-accuracy variational benchmarks, we derive the exact conditions for numerical equivalence between Ω and ΛS formulations and quantify how missing NAC terms and bond-length-dependent spin factors degrade predictions. We implement a complete Ω-representation workflow in Duo for diatomics, fully transforming all Hamiltonian terms and enabling side-by-side Ω versus ΛS calculations. For common single-state pipelines (e.g., LEVEL), we provide diagnostics that flag unsafe regimes and practical remedies to restore accuracy. The results deliver actionable guidance for spectroscopy, photophysics, and kinetics: Ω-based single-state approximations are reliable only when interacting states are well separated in the Franck–Condon region; otherwise, explicit nonadiabatic terms are required─even for “forbidden” transitions.