The ground state vinylidene-acetylene isomerization was investigated by ab initio molecular electronic structure theory. The coupled-cluster method with single, double, and noniterative inclusion of triple excitations [CCSD(T)]; with single, double, and noniterative inclusion of triple and quadruple excitations [CCSD(TQ)]; and with full single, double, and triple excitations (CCSDT) were used to treat the effect of electron correlation. Several correlation-consistent polarized valence basis sets, cc-pVXZ, were employed. Theoretical limiting values of the energetics of the reaction were then deduced from the series of computations. With zero-point energy correction, the energy of reaction is -42.95 kcal/mol and the reaction barrier is 1.5 kcal/mol. Both agree excellently with experimental values.