HC + D Û H + CD

This screen shows the trajectory on the potential energy surface of the exchange reaction HC + D Û H + CD with a stable intermediate state H..C..D. After many vibrations the triatomic state decays towards the entry or exit valley of the reaction if it has sufficient kinetic energy. The chosen initial positions of the nuclei and their momenta lead to a successful isotope substitution. You can change those to view entirely different trajectories. A very small change of the order of 0.000'001 of one of the momenta can reverse the reaction and send the system back to the entry channel. This is akin to chaotic behavior at the microscopical level. Potential for HCH from R.F. Nalewajski & R.E. Wyatt, Chem.Phys 81(1983)357 changed to HCD. Coordinate system is mass-weighted and the skew angle diagonalizes the kinetic energy. The actual program shows the system point in motion, of course.