There is a tendency to describe dispersion interactions as weak interactions. (Sometimes the phrase "van der Waals interactions" is used to mean dispersion, but we avoid that terminology.) Dismissing dispersion as a weak effect is prevalent in the literature on drug discovery and noncovalent enzyme inhibition, where "vdW interactions" are sometimes described as sub-kcal/mol phenomenon. 213 This is true for the raregas dimers that are often used to introduce London dispersion, as evident from the Ar 2 data in Fig. . However, the magnitude of E disp scales with system size and need not be small in in large molecular systems, even as compared to covalent bond energies of ∼ 100 kcal/mol. Assuming typical values for atomic C 6 coefficients, a rough estimate suggests that two 100-atom organic molecules at 5 Å separation might exhibit a mutual dispersion interaction E disp ∼ -60 kcal/mol. Entropic effects tend to reduce interaction energies through entropy/enthalpy compensation, such that |∆G • bind | < 20 kcal/mol even for ∼ 100-atom host-guest complexes for which |∆U • bind | exceeds 100 kcal/mol. 169 Nevertheless, in order to understand the balance of intermolecular forces it is important to get the interaction energies right. Qualitative effects of dispersion on organic reactivity have been discussed, aided by in silico computational modeling in which dispersion interactions can be turned off in order to isolate their effects. This has led to the fascinating idea of "steric attraction", in which a bulky substituent also contributes substantially to the polarizability, enhancing E disp to the point that it may outcompete E exch and the larger substituent becomes the less repulsive one. This phenomenon is only possible because dispersion is size-extensive.