  The symmetry energy in the nuclear equation of state affects many aspects of
nuclear astrophysics, nuclear structure, and nuclear reactions. Recent
constraints from heavy ion collisions, including isospin diffusion observables,
have started to put constraints on the symmetry energy below nuclear saturation
density, but these constraints depend on the employed transport model and input
physics other than the symmetry energy. To understand these dependencies, we
study the influence of the symmetry energy, isoscaler mean field
compressibility and momentum dependence, in-medium nucleon-nucleon cross
sections, and light cluster production on isospin diffusion within the pBUU
transport code. In addition to the symmetry energy, several uncertain issues
strongly affect isospin diffusion, most notably the cross sections and cluster
production. In addition, there is a difference in the calculated isospin
transport ratios, depending upon whether they are computed using the isospin
asymmetry of either the residue or of all forward moving fragments.
Measurements that compare the isospin transport ratios of these two quantities
would help place constraints on the input physics, such as the density
dependence of the symmetry energy.
