We study the chemisorption of CO molecule into sites of different coordination on ͑111͒ surfaces of late 4d and 5d transition metals. In an attempt to solve the well-known CO adsorption puzzle, i.e., discrepancies of adsorption site preferences with experiment which appear in the standard density functional theory calculations, we have applied the relatively new van der Waals-density functional of nonlocal correlation. In all considered cases this reduces or completely solves the site preference discrepancies and improves the value of the adsorption energy. By introducing a cutoff distance for nonlocal interaction we can pinpoint the length scale at which the correlation plays a major role in the systems considered.

We have studied the high-pressure cubic fluorite polymorph of TiO2 (c-TiO2) using the diffusion Monte Carlo (DMC) method. The estimated bulk modulus is within the range reported previously in density functional studies, high, but does not rival that of diamond. The calculated excitation energies within DMC are consistent with the results of GW approximation. The infrared frequency of c-TiO2, obtained via the frozen phonon method within DMC, shows non-negligible anharmonicity. This suggests that c-TiO2 might be stabilized if this anharmonicity is considered. Our DMC results could help to establish more accurate results for c-TiO2 compared with the widely-scattered mean-field results.

FeF3, with its half-filled Fe 3+ 3d orbital, hence zero orbital angular momentum and S = 5/2, is often put forward as a prototypical highly-frustrated classical Heisenberg pyrochlore antiferromagnet. By employing ab initio density functional theory (DFT), we obtain an effective spin Hamiltonian for this material. This Hamiltonian contains nearest-neighbor antiferromagnetic Heisenberg, bi-quadratic and Dzyaloshinskii-Moriya interactions as dominant terms and we use Monte Carlo simulations to investigate the nonzero temperature properties of this minimal model. We find that upon decreasing temperature, the system passes through a Coulomb phase, composed of shortrange correlated coplanar states, before transforming into an "all-in/all-out" (AIAO) state via a very weakly first order transition at a critical temperature Tc ≈ 22 K, in good agreement with the experimental value for a reasonable set of Coulomb interaction U and Hund's coupling JH describing the material. Despite the transition being first order, the AIAO order parameter evolves below Tc with a power-law behavior characterized by a pseudo "critical exponent" β ≈ 0.18 in accord with experiment. We comment on the origin of this unusual β value.

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