  We apply the envelope function approach to abrupt heterostructures starting
with the least action principle for the microscopic wave function. The
interface is treated nonperturbatively, and our approach is applicable to
mismatched heterostructure. We obtain the interface connection rules for the
multiband envelope function and the short-range interface terms which consist
of two physically distinct contributions. The first one depends only on the
structure of the interface, and the second one is completely determined by the
bulk parameters. We discover new structure inversion asymmetry terms and new
magnetic energy terms important in spintronic applications.
