The stochastic computer simulation of ion motion in a gas subjected to a constant electric field

The stochastic computer simulation of ion motion in a gas subjected to an electric field involves the problem of statistically choosing free times between the collisions, when the velocity cannot be assumed constant during a free time. It is shown how free times can be chosen, without any approximations or numerical integrations, by the use of a piecewise velocity-independent mean collision frequency chosen to be greater than the actual collision frequency, combined with the assumption that an appropriate part of the collisions results in zero momentum transfer.