  Potassium intercalation in graphite is investigated by first-principles
theory. The bonding in the potassium-graphite compound is reasonably well
accounted for by traditional semilocal density functional theory (DFT)
calculations. However, to investigate the intercalate formation energy from
pure potassium atoms and graphite requires use of a description of the graphite
interlayer binding and thus a consistent account of the nonlocal dispersive
interactions. This is included seamlessly with ordinary DFT by a van der Waals
density functional (vdW-DF) approach [Phys. Rev. Lett. 92, 246401 (2004)]. The
use of the vdW-DF is found to stabilize the graphite crystal, with crystal
parameters in fair agreement with experiments. For graphite and
potassium-intercalated graphite structural parameters such as binding
separation, layer binding energy, formation energy, and bulk modulus are
reported. Also the adsorption and sub-surface potassium absorption energies are
reported. The vdW-DF description, compared with the traditional semilocal
approach, is found to weakly soften the elastic response.
