Intermolecular dynamics can be described in the context of quantum mechanics by the movement of nuclear wave packets on the associated electronic potential surfaces. If, for example, you want to look at the acid-base reaction between hydrochloric acid and fluorine, you first calculate the 2D electronic potential surface, which depends on the distances between chlorine & hydrogen and fluorine & hydrogen. If you then place a wave packet, which describes the dynamics of the atomic nuclei, on this potential surface, it moves in the direction of certain new nuclear distances, depending on the starting conditions. Depending on whether at the end of the temporal dynamics the wave packet is located on the potential at the location of small distances between Cl & H or F & H, a transition of the hydrogen atom from chlorine to fluorine (acid-base reaction) has taken place or not.

The following video shows the dynamics of such a wave packet (shaded blue) on the potential surface (shaded purple). The wave packet starts at the location of the minimum distance between H & Cl and the maximum distance between H & F. In the course of the dynamics, the wave packet moves towards a small distance between H & F, while the distance between H & Cl remains constant. At the edge of the potential, the wave packet is reflected and returns to its starting position. The starting condition therefore excludes a chemical reaction.

If the HCl molecule is subjected to a small vibration at the beginning (as can be seen from the movement of the wave packet along the H-Cl distance), a reaction takes place and the wave packet drifts in the direction of small H-F distances or large H-Cl distances. The initial vibration is retained, but is now mapped onto the H-F distance axis.