The use and misuse of van der Waals radii

A widely used criterion for the existence of a noncovalent interaction is that the interatomic separation be less than the sum of the van der Waals radii of the respective atoms. However, this criterion should not be applied rigidly, but rather with considerable caution and flexibility, for several reasons. First, there is considerable uncertainty in the values assigned as van der Waals radii. Second, because of the manner in which they are assigned, their sums will necessarily miss a significant number of noncovalent interactions. Finally, not all interactions are between atoms; sometimes, instead of an atom, it is a region of positive or negative electrostatic potential that is involved.