When performing electronic structure calculations on a molecule one can surround it with point charges to mimic a solvent environment and lend polarization effects. There is a bit of a caveat unfortunately.
If diffuse functions are used, there is a risk that the point charges surrounding the "cavity" that the molecule of interest is in, will lead to distortion of the electron density. Another way of saying this is that the electron density will(may) migrate to towards the point charges in an unphysical manner.
This is unfortunate, but, not too big of a deal for neutral and cation species which are not reliant on diffuse functions (just don't use them). However, anions really should be done with diffuse functions to account for the loose electron. This applies to anything where electron density will not be as tight around nuclei, i.e., excited states.
Is there a ready solution to this problem of charge migration?
This also applies when using implicit solvents as well, the tessellated surface of the cavity can lead to unphysical migration of the solute's electron density towards the cavity surface.