  We show that the highly excited rovibrational spectra of a diatomic molecule
and the closely related slow atomic collision processes contain more
systematics and require less parameters to characterize than the Rydberg
spectrum of an atom. In the case of a single channel, for example, we show that
a single short-range parameter gives a complete description of slow collisions
for practically all angular momenta, and covers an energy range of hundreds of
millikelvins. The same parameter also describes the highly excited
rovibrational spectra in the threshold region, including states of different
angular momenta. Sample applications and predictions of the theory are
discussed, including comparisons with experiment.
