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Only a limited number of diatomic interactions have been parameterized. These cover the more popular types of system, e.g., interactions involving C, N, O, halogen, P, and S, and interactions involving an atom of an element with another atom of the same element. Most possible interactions are not parameterized. For such interactions, a crude approximation is used. This is good enough for systems in which the unparameterized interaction involves atoms that are not near to each other, but would likely give bad results if the atoms are near to each other.
Dispersion terms are only used for atom pairs involving atoms of the elements: H, He, B, C, N, O, F, Ne, P, S, Cl, Ar, Co, Br, Kr, I, and Xe. All other elements do not have dispersion terms, and therefore lack any Van der Waals type interactions.