  Given the difficulty of obtaining compact analytical solutions for diffusion
of interacting geminate pairs (such as electron-hole pairs generated by
ionization of liquid) it is common, following the original method of Mozumder,
to "prescribe" this diffusion. With this approach, the survival probability of
the pair is represented as a product of the survival probability for a freely
diffusing pair and a suitably defined weighting function. This approach gives
the correct limiting survival probability for a pair in the Coulomb field. The
same approach was used for simulation of reaction dynamics in radiolytic spurs
("independent reaction times" approach of Pimblott) and solve other vexing
diffusion problems that do not have analytical solution. A reasonable question
is, can the same method be used for any other interaction potential than
Coulomb? Here we demonstrate that such a prescription is generally impossible.
The correct result given by the prescribed diffusion approach for the Coulomb
potential is, actually, purely accidental. The method is inherently
contradictory and it should be used with caution.
