  The numerically exact evaluation of the van der Waals interaction, also known
as Casimir interaction when including retardation effects, constitutes a
challenging task. We present a new approach based on the plane-wave basis and
demonstrate that it possesses advantages over the more commonly used multipole
basis. The rotational symmetry of the plane-sphere and sphere-sphere geometries
can be exploited by means of a discrete Fourier transform. The new technique is
applied to a study of the interaction between a colloid particle made of
polystyrene or mercury and another polystyrene sphere or a polystyrene wall in
an aqueous solution. Special attention is paid to the influence of screening
caused by a variable salt concentration in the medium. It is found that in
particular for low salt concentrations the error implied by the proximity force
approximation is larger than usually assumed. For a mercury droplet, a
repulsive interaction is found for sufficiently large distances provided
screening is negligible. We emphasize that the effective Hamaker parameter
depends significantly on the scattering geometry on which it is based.
