  Theories for the effective polarisability of a small particle in a medium are
presented using different levels of approximation: we consider the virtual
cavity, real cavity and the hard-sphere models as well as a continuous
interpolation of the latter two. We present the respective hard-sphere and
cavity radii as obtained from density-functional simulations as well as the
resulting effective polarisabilities at discrete Matsubara frequencies. This
enables us to account for macroscopic media in van der Waals interactions
between molecules in water and their Casimir-Polder interaction with an
interface.
