  The dissociation energies of four transition metal dimers are determined
using diffusion Monte Carlo. The Jastrow, CI, and molecular orbital parameters
of the wave function are both partially and fully optimized with respect to the
variational energy. The pivotal role is thereby ascribable to the optimization
of the molecular orbital parameters of a complete active space wave function in
the presence of a Jastrow correlation function. Excellent results are obtained
for ZnO, FeO, FeH, and CrS. In addition, potential energy curves are computed
for the first three compounds at multi-reference diffusion Monte Carlo (MR-DMC)
level, from which spectroscopic constants such as the equilibrium bond
distance, the harmonic frequency, and the anharmonicity are extracted. All of
those quantities agree well with the experiment. Furthermore, it is shown for
CrS that a restricted active space calculation can yield improved initial
orbitals by including single and double excitations from the original active
space into a set of virtual orbitals. We demonstrated in this study that the
fixed-node error in DMC can be systematically reduced for multi-reference
systems by orbital optimization in compact active spaces. While DMC
calculations with a large number of determinants are possible and very
accurate, our results demonstrate that compact wave functions may be sufficient
in order to obtain accurate nodal surfaces, which determine the accuracy of
DMC, even in the case of transition metal compounds.
