Quantum defect theory with deeply closed channels

We address the validity and practicality of using multi-channel quantum def ect theory (MQDT) to treat electron-ion collision problems for cases where some channel solutions propagate with extremely negative, classically forbi dden, energies. We discuss theoretically and demonstrate numerically, withi n an unrealistically sensitive model-problem study, how MQDT results can be obtained which are in agreement with those obtained from the more standard non-MQDT method at all energies.