It is remarkable how many people think it difficult to compute van der Waals forces directly by using the Lifshitz theory. Invariably, after a few minutes' instruction, there is the reaction “I didn't know how easy it was.” Essentially it is a matter of introducing tabulated experimental information for ε's and numerically summing or integrating for the interaction energy.
Thanks to rapid progress in spectroscopy we can soon expect to compute van der Waals forces by direct conversion of material responses to applied electromagnetic fields. The future of precise computation relies on measuring these responses on the same materials as those used to measure forces. This is because small changes—in composition of materials, dopants that confer conductance, solutes that modify spectra, even in atomic arrangement that create anisotropies—all have quantitative consequences. Whether the unintended result of handling materials or artful modifications to create forces, spectral details merit respect.
Combining these data with the theory of dielectrics helps us to think about the connection between specific features of absorption spectra and forces in order to create strategies for designing materials or to find reasons to explain measured forces. Outside the purview of this text, the actual practice of assembling data into usable ε(ω)'s and ε(iε)'s has been well described in the physics and the engineering literature. To connect readers with that literature, this chapter first introduces the essential physical features and language used to create ε's of computation.