We report ab initio potential energy and dipole moment surfaces that span the regions describing the minima of trans- and cis-HOC and the barrier separating them. We use the new potential in three types of variational calculations of the vibrational eigenstates, for zero total angular momentum Two use the code MULTIMODE (MM) in the socalled single-reference and reaction path versions. The third uses the exact Hamiltonian in diatom-diatom Jacobi coordinates The single reference version of MM is limited to a description of states that are localized at each minimum separately, whereas the reaction path version and the Jacobi approach describe localized and delocalized states. The vibrational IR spectrum for zero total angnlar, momentum is also reported for the trans and cis fundamentals and selected overtone and combination states with significant oscillator strength.