  Although the precise microscopic knowledge of van der Waals interactions is
crucial for understanding bonding in weakly bonded layered compounds, very
little quantitative information on the strength of interlayer interaction in
these materials is available, either from experiments or simulations. Here,
using many-body perturbation and advanced density-functional theory techniques,
we calculate the interlayer binding and exfoliation energies for a large number
of layered compounds and show that, independent of the electronic structure of
the material, the energies for most systems are around 20 meV/\AA$^2$. This
universality explains the successful exfoliation of a wide class of layered
materials to produce two-dimensional systems, and furthers our understanding
the properties of layered compounds in general.
