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As gold clusters increase in size, the preferred structure changes from planar to three-dimensional and, for anionic clusters, Aun-, the two-dimensional(2D)→three-dimensional (3D) transition is found experimentally to occur between n=11 and n=12. Most density functionals predict that planar structures are preferred up to higher n than is observed experimentally, an exception being the local spin density approximation. Here we test four relatively new functionals for this feature, in particular, M05, M06-L, M06, and SOGGA. We find that M06-L, M06, and SOGGA all predict the 2D→3D transition at the correct value of n. Since the M06-L and M06 functionals have previously been shown to be reasonably accurate for transition metal bond energies, main group atomization energies, barrier heights, and noncovalent interaction energies, and, since they are here shown to perform well for the s-d excitation energy and ionization potential of Au atoms and for the size of Aun- clusters at which the 2D→3D transition occurs, they are recommended for simulating processes catalyzed by gold clusters. [ABSTRACT FROM AUTHOR]

The adsorption of CO on Mg(001) constitutes a challenge for current density functional approximations because of its weak interaction character. In the present work we show that the M06-2X and M06-HF exchange-correlation functionals are the first ones to provide a simultaneously satisfactory description of adsorbate geometry, vibrational frequency shift, and adsorption energy of CO on MgO(001). For a sufficiently large embedded cluster model, the three functionals of the M06 family—which contain a nonzero percentage of Hartree–Fock exchange (M06, M06-2X, and M06-HF)—all predict positive C–O vibrational shifts, in agreement with the experimental findings, while the local M06-L functional gives large negative shifts. Moreover, the shifts computed with the M06-2X and M06-HF potentials are in good agreement with the experimental shift of +14 cm-1. The interaction energy (De) calculated with M06-2X and M06-HF is ∼6.0 kcal/mol, which agrees well with the De value (∼4 kcal/mol) deduced from the D0 obtained in thermal desorption measurements on single-crystal surfaces. [ABSTRACT FROM AUTHOR]

The performance of the M06 family of exchange-correlation potentials for describing the electronic structure and the Heisenberg magnetic coupling constant (J) is investigated using a set of representative open-shell systems involving two unpaired electrons. The set of molecular systems studied has well defined structures, and their magnetic coupling values are known experimentally. As a general trend, the M06 functional is about equally as accurate as B3LYP or PBE0. The performance of local functionals is important because of their economy and convenience for large-scale calculations; we find that M06-L local functional of the M06 family largely improves over the local spin density approximation and the generalized gradient approximation. [ABSTRACT FROM AUTHOR]