“…We study the spatially homogeneous case with vanishing electric field, but we construct the multigroup equations in a way that they allow the description of particle distributions of arbitrary anisotropy with respect to a main direction imposed, for example, by an electric field. Hence, the transport model can easily be extended to describe field dependent, spatially inhomogeneous problems by including multigroup formulations of the advection terms in the BBP equations following the procedures given in [7], [8]. A deterministic transport model is the more reliable, the more mathematical properties the model equations and the original BBP equations possess in common grounds.…”