  Isomerization between the cis and trans conformers of the S1 state of
acetylene is studied using a reduced dimension DVR calculation. Existing DVR
techniques are combined with a high accuracy potential energy surface and a
kinetic energy operator derived from FG theory to yield an effective but simple
Hamiltonian for treating large amplitude motions. The spectroscopic signatures
of the S1 isomerization are discussed, with emphasis on the vibrational
aspects. The presence of a low barrier to isomerization causes distortion of
the trans vibrational level structure and the appearance of nominally
electronically forbidden \~A1A2 \leftarrow X 1{\Sigma}+g transitions to
vibrational levels of the cis conformer. Both of these effects are modeled in
agreement with experimental results, and the underlying mechanisms of tunneling
and state mixing are elucidated by use of the calculated vibrational
wavefunctions.
