Multichannel Quantum Defect Theory for cold molecular collisions with a strongly anisotropic potential energy surface

We show that multichannel quantum defect theory (MQDT) can be applied successfully as an efficient computational method for cold molecular collisions in Li+NH, which has a deep and strongly anisotropic interaction potential. In this strongly coupled system, closed-channel poles restrict the range over which the MQDT Y matrix can be interpolated. We present an improved procedure to transform the MQDT reference functions so that the poles are removed from the energy range of interest. Effects due to very long-range spin-dipolar couplings are outside the scope of MQDT, but can be added perturbatively. The new procedure makes it possible to calculate the elastic and inelastic cross sections needed to evaluate the feasibility of sympathetic cooling of NH by Li using coupled-channel calculations at only 5 combinations of energy and field.