A Quantum Monte Carlo Study of the Ground State Chromium Dimer

We take variational and diffusion quantum Monte Carlo (VMC and DMC) methods to investigate the chemical binding of the ground state chromium dimer, employing various single determinant (SD) or multi-determinant (MD) wavefunctions multiplied by a Jastrow factor as a trial/guiding wavefunction. The molecular orbitals (MOs) entering the SD wavefunction were calculated using restricted or unrestricted Hartree-Fock or density functional theory (DFT) calculations where five commonly-used local (SVWN5), semi-local (PW91 and BLYP), and hybrid (B1LYP and B3LYP) functionals were examined. The MD expansions were obtained from the complete-active space self-consistent field, generalized valence bond, and unrestricted configuration interaction methods. We also adopted the UB3LYP MOs to construct the MD expansion and optimized their coefficients at the VMC level. 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 the DMC simulations. (C) 2012 American Chemical Society