  Ultracold collisions of cold atoms or molecules make the bound states of the
collision complex formed from the two colliding species accessible for control
and manipulation of the cold species or the complex. Such resonances are best
treated by a resonant scattering theory, which in the ultracold domain can take
advantage of the properties of the long-range potential and the methods of
multichannel quantum defect theory. Coupled channels calculations on the
threshold scattering states and bound states of the 40K87Rb molecule illustrate
the ideas and methodology of quantum defect theory using the long-range
potential and also demonstrate the spin properties of the bound states
throughout the spectrum.
