  Rearrangement mechanisms of the water dimer and the cage form of the water
hexamer are examined theoretically with particular reference to tunneling
splittings and spectroscopy. The three lowest barrier rearrangements of the
water dimer are characterized by {\it ab initio} methods and compared with the
results of previous constrained calculations. The acceptor-tunneling pathway
does not proceed via a direct rotation around the $C_2$ axis of the acceptor,
but rather via relatively asynchronous rotation of the donor about the hydrogen
bond and an associated `wag' of the acceptor. Rearrangements between different
cage isomers of the water hexamer are studied for two empirical potentials. The
experimentally observed triplet splittings may be the result of flip and
bifurcation rearrangements of the two single-donor, single-acceptor monomers.
Two-dimensional quantum calculations of the nuclear dynamics suggest that
delocalization over more than one cage isomer may occur, especially in excited
states.
