  We have developed an 8-band Effective Mass Approximation model that describes
the zero field spin splitting in the band structure of zincblende
heterostructures due to bulk inversion asymmetry (BIA). We have verified that
our finite difference Hamiltonian transforms in almost all situations according
to the true $D_{2d}$ or $C_{2v}$ symmetry of [001] heterostructures. This makes
it a computationally efficient tool for the accurate description of the band
structure of heterostructures for spintronics. We first compute the band
structure for an AlSb/GaSb/AlSb quantum well (QW), which presents only BIA, and
delineate its effects. We then use our model to find the band structure of an
AlSb/InAs/GaSb/AlSb QW and the relative contribution of structural and bulk
inversion asymmetry to the spin splitting. We clarify statements about the
importance of these contributions and conclude that, even for our small gap QW,
BIA needs to be taken into account for the proper description of the band
structure.
