Diffusion Monte Carlo (DMC) calculations are performed on the monocyclic and bicyclic forms of m -benzyne, which are the equilibrium structures at the CCSD(T) and CCSD levels of coupled cluster theory. We employed multiconfiguration self-consistent field trial wave functions which are constructed from a carefully selected eight-electrons-in-eight-orbitals complete active space [CAS(8,8)], with configuration state function coefficients that are reoptimized in the presence of a Jastrow factor. The DMC calculations show that the monocyclic structure is lower in energy than the bicyclic structure by 1.9 (2) kcalmole, which is in excellent agreement with the best coupled cluster results. © 2008 American Institute of Physics.