  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
multi-configuration self-consistent field trial wave functions which are
constructed from a carefully selected 8-electrons-in-8-orbitals complete active
space [CAS(8,8)], with CSF 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) kcal/mole, in excellent
agreement with the best coupled cluster results.
