We present ab initio calculations of the interlayer exchange coupling between two, in general noncollinearly aligned magnetic slabs embedded in a nonmagnetic spacer. Based on a surface Green’s function formalism, two equivalent but formally and physically different approaches are examined and discussed. For the Co/Cu/ Co~001! system we demonstrate the usefulness of the concept of infinitesimal rotations in order to calculate the coupling for a finite relative angle u, in particular for u5p, between the corresponding spin directions in the magnetic slabs. The temperature and layer dependence of the interlayer exchange coupling is examined and the possibility for noncollinear coupling is investigated. @S0163-1829~96!02121-2#