The *cis-trans* isomerization reaction on the
T_{2} surface of acetylene and the lowest excited
singlet state of acetylene, A ^{1}A_{u},
are investigated by *ab initio* electronic structure
theory. We report optimized geometries, dipole moments, and
harmonic vibrational frequencies of stationary points and
adiabatic energy differences between them using basis sets
as large as triple-zeta plus double polarization with
higher angular momentum functions, TZ(2df,2pd), and
theoretical methods up to coupled-cluster singles and
doubles with perturbative triples correction [CCSD(T)] and
the equation-of-motion coupled-cluster method (EOM-CCSD).
Our theoretical predictions should aid the interpretation
of observations from a series of recent spectroscopic
studies involving excited electronic states of acetylene.
In particular, the present theoretical results rule out
several possible explanations for the anomalous sudden
increase in detectable Zeeman anticrossings reported by
Dupre *et al.* [Chem. Phys. **152**, 293 (1991)].