  We report variational and diffusion quantum Monte Carlo (VMC and DMC) studies
of the binding curve of the ground-state chromium dimer. We employed various
single determinant (SD) or multi-determinant (MD) wavefunctions multiplied by a
Jastrow fuctor as a trial/guiding wavefunction. The molecular orbitals (MOs) in
the SD were calculated using restricted or unrestricted Hartree-Fock or density
functional theory (DFT) calculations where five commonly-used local (SVWN5),
semi-local (PW91PW91 and BLYP), and hybrid (B1LYP and B3LYP) functionals were
examined. The MD expansions were obtained from the complete-active space SCF,
generalized valence bond, and unrestricted configuration interaction methods.
We also adopted the UB3LYP-MOs to construct the MD expansion (UB3LYP-MD) and
optimized their coefficients at the VMC level. In addition to the wavefunction
dependence, we investigated the time-step bias in the DMC calculation and the
effects of pseudopotentials and backflow transformation for the UB3LYP-SD case.
Some of the VMC binding curves show a flat or quite shallow well bottom, which
gets recovered deeper by DMC. All the DMC binding curves have a minimum
indicating a bound state, but the comparison of atomic and molecular energies
gives rise to a negative binding energy for all the DMC as well as VMC
calculations.
