  The depletion interactions between two colloidal plates or between two
colloidal spheres, induced by interacting polymers in a good solvent, are
calculated theoretically and by computer simulations. A simple analytical
theory is shown to be quantitatively accurate for case of two plates. A related
depletion potential is derived for two spheres; it also agrees very well with
direct computer simulations. Theories based on ideal polymers show important
deviations with increasing polymer concentration: They overestimate the range
of the depletion potential between two plates or two spheres at all densities,
with the largest relative change occurring in the dilute regime. They
underestimate the well depth at contact for the case of two plates, but
overestimate it for two spheres. Depletion potentials are also calculated using
a coarse graining approach which represents the polymers as ``soft colloids'':
good agreement is found in the dilute regime. Finally, the effect of the
polymers on colloid-colloid osmotic virial coefficients is related to phase
behavior of polymer-colloid mixtures.
