  Pairs of trapped atoms can be associated to make a diatomic molecule using a
time dependent magnetic field to ramp the energy of a scattering resonance
state from above to below the scattering threshold. A relatively simple model,
parameterized in terms of the background scattering length and resonance width
and magnetic moment, can be used to predict conversion probabilities from atoms
to molecules. The model and its Landau-Zener interpretation are described and
illustrated by specific calculations for $^{23}$Na, $^{87}$Rb, and $^{133}$Cs
resonances. The model can be readily adapted to Bose-Einstein condensates.
Comparison with full many-body calculations for the condensate case show that
the model is very useful for making simple estimates of molecule conversion
efficiencies.
