The isomerization of acetylene to vinylidene is examined theoretically in full dimensionality (six degrees of freedom), using a new ab initiopotential energy surface [S. Zou and J. M. Bowman, Chem. Phys. Lett. 368, 421 (2003)]. Eigenfunctions and eigenvalues of the exact Hamiltonian, for zero total angular momentum,are obtained using a series of novel truncation/recoupling procedures that permits calculations up to very high energies. The Hamiltonian is given in diatom–diatom Jacobi coordinates, with the choice H2–C2 for the two diatoms in order to exploit the full permutational symmetry of the problem. By examining expectation values of the eigenfunctions, a number of states are definitely identified with vinylidenelike characteristics. Corresponding calculations are also done for C2D2. Full dimensional simulations of the photodetachment spectra of C2H2− and C2D2− are done (within the Franck–Condon approximation) and compared to the experimental ones. For this calculation the ground vibrational state wave function of the anion is obtained using a new force field, based on high quality ab initio calculations, which are also briefly reported.