Infinite Free-Energy Barrier Disrupts Key Chemical Reaction Simulations

A new study has uncovered why molecular simulations often fail to capture key chemical reactions at conical intersections. Researchers at the University of Vienna found an infinite free-energy barrier blocking trajectories in mixed quantum-classical models. This barrier prevents accurate predictions of photochemical processes, even when simulations approach critical reaction zones.

Conical intersections are known as vital pathways for nonadiabatic transitions, where molecules switch between electronic states. However, simulations frequently miss these exact points, leading to incomplete predictions. The team—Johannes C. B. Dietschreit, Sebastian Mai, and Leticia González—used a linear vibronic coupling model to investigate the issue.

Their analysis revealed an infinite free-energy barrier surrounding the conical intersection in the methanimine cation (CH₂NH₂⁺). This barrier causes trajectories to reflect rather than cross, severely limiting the accuracy of photochemical dynamics simulations. Molecular dynamics tests on the S₁ surface confirmed the theory: trajectories approached small adiabatic gaps but never reached the intersection seam.

The findings build on earlier work using the Landau-Zener formula, which showed that trajectory behaviour depends on approach direction and local topography. The study also aligns with recent evidence suggesting classical trajectories can 'sense' conical intersections without directly visiting them. By quantifying the free-energy barrier, the researchers explained why mixed quantum-classical methods still predict nonadiabatic behaviour—transitions occur in regions of strong coupling, not at exact degeneracies.

The discovery clarifies a long-standing challenge in molecular dynamics simulations. The infinite free-energy barrier explains why trajectories avoid conical intersections, yet nonadiabatic transitions still take place. This insight improves the reliability of mixed quantum-classical models for predicting photochemical reactions and product yields.