The Phase Space Time Evolution (PSTE) method was initially developed for one speed neutrons. This discussion considers the alterations performed in transforming the one speed case to a more general multigroup code. Also the complications which arose in this transformation are discussed.

The multigroup formulation of the PSTE method calculates the energy dependent scalar and angular neutron density at very small time increments. In a reactor even slow neutrons travel at relatively high velocities and therefore to keep the distance the particles travel on the order of their mean free path, a small time increment must be used.

To illustrate the usefulness of the PSTE method, the time evolution of the neutron density of a nuclear device is modeled in slab geometry and the energy dependent scalar and angular flux is displayed as a function of time and space.