Ab initio molecular electronic structure theory is used to study the electronic ground state vinylidene-acetylene isomerization. Vinylidene, acetylene, and the transition state connecting them are located at various levels of theory, including correlated levels, and with large basis sets. The highest level and basis set with which geometry optimizations are performed is the CCSD level with the TZ+2P basis set. These structures are characterized by harmonic vibrational analyses as minima or transition states. Single-point energies also are computed at higher levels of theory, the highest being CCSD/QZ+3P, forall three structures. The effects of carbon atom f functions and hydrogen atom d functions are also explicitly considered. Aclassical barrier to isomerization from vinylidene to acetylene of ∼3 kcal/mol is found. The ΔE for isomerization is predictedto be ∼43 kcal/mol. © 1990, American Chemical Society. All rights reserved.