  This paper improves the convergence and robustness of a multigrid-based
solver for the cross sections of the driven Schroedinger equation. Adding an
Coupled Channel Correction Step (CCCS) after each multigrid (MG) V-cycle
efficiently removes the errors that remain after the V-cycle sweep. The
combined iterative solution scheme (MG-CCCS) is shown to feature significantly
improved convergence rates over the classical MG method at energies where bound
states dominate the solution, resulting in a fast and scalable solution method
for the complex-valued Schroedinger break-up problem for any energy regime. The
proposed solver displays optimal scaling; a solution is found in a time that is
linear in the number of unknowns. The method is validated on a 2D Temkin-Poet
model problem, and convergence results both as a solver and preconditioner are
provided to support the O(N) scalability of the method. This paper extends the
applicability of the complex contour approach for far field map computation [S.
Cools, B. Reps, W. Vanroose, An Efficient Multigrid Calculation of the Far
Field Map for Helmholtz and Schroedinger Equations, SIAM J. Sci. Comp. 36(3)
B367--B395, 2014].
