Van der Waals molecules are complexes held together by the weak longrange forces between closed-shell atoms and/or molecules, i.e., electrostatic forces, dispersive forces, hydrogen bonding, and charge transfer interactions (Van der Avoird, Wormer, Mulder, and Berns 1980; Buckingham, Fowler, and Hutson 1988). Typical examples are X … I2, X … IC1, X … Cl2, X … HF, HF … HF with X being He, Ne, Ar, etc. The dots indicate the weak physical bonding in contrast to a strong chemical bonding.

The main characteristics of van der Waals molecules are:

1. 1) The small dissociation energy ranging from a few cm−1 to about 1000 cm−1.

2. 2) The relatively large bond length, Re, of typically 4 Å.

3. 3) The retention of the properties of the individual entities within the van der Waals complex.

4. 4) Relatively weak coupling between the van der Waals mode and the internal coordinates of the molecular entity.

Owing to the small dissociation energy, van der Waals molecules exist mainly at very low temperatures as they prevail in the interstellar medium or in supersonic jets.

Van der Waals molecules are rather floppy complexes which can be best described by the usual Jacobi coordinates which we used throughout all previous chapters (see the inset of Figure 12.1): the van der Waals bond distance R, the intramolecular separation of the chemically bound molecule r, and the orientation angle γ. Figure 12.1 depicts the R dependence of a typical potential energy surface (PES) V(R, r, γ) for fixed values of r and γ. The long-range part of the potential is attractive and governed by the forces mentioned above.