ABSTRACT
The correct description of van der Waals (vdW) interaction forces is required for accurately describing dispersion bonded systems. Several approaches have been proposed to include London dispersion in density functional theory exchange-correlation functionals, where the family of so-called van der Waals (vdW-DF) exchange-correlation functionals have shown a better performance than local or semi local exchange-correlation functionals for describing molecular adsorption on metals. Despite the numerous benchmarks performed with these functionals, their performance in predicting bulk properties of transition metals has hitherto not been investigated in detail. We have therefore tested five vdW-DF exchange-correlation functionals, vdW-DF2, optPBE-vdW, BEEF-vdW, optB88-vdW and C09 x -vdW to assess their performance in the prediction of lattice constants, bulk moduli, cohesive energies and surface energies of bulk Ni, Cu, Rh, Pd, Ag, Ir, Pt and Au (in fcc crystal structure). These transition metals are commonly used for benchmarking density functionals because they are important for applications in catalysis. The results are compared with experimental data and the PBE exchange-correlation functional. We found that both the optB88-vdW and the C09 x -vdW exchange-correlation functionals estimate all properties with high accuracy, in better agreement with experimental data than PBE and other considered vdW functionals. The C09 x -vdW functional clearly outperforms all other exchange-correlation functionals for surface energies for the (1 1 1) termination of different metals. We have also evaluated the interatomic electron density emerging from different functionals, and concluded that the observed differences are a result of the predicted lattice parameter, rather than a direct consequence of the functional form. Plane-wave and real-space grid-based expansions of the electron density are also compared, revealing good agreement between the two approaches for lattice parameters, cohesive energies, and surface energies, but more severe differences in bulk moduli. On the basis of our results, we recommend using the C09 x -vdW for studying bulk properties and surface energies of transition metals.
ABSTRACT
Eleven adducts for the interaction between imidacloprid (IMI) and some activated carbon (AC) pieces are proposed in this work. Activated carbon pieces were obtained by using a finite zig-zag graphene structure saturated with hydrogen atoms on the edges giving a pristine model with 70 carbon atoms and 22 hydrogen atoms. The zig-zag graphene structure was oxidized with -O, -COOH, -OH, and -O- groups. In this process, two identical groups were inserted over selected sites of the pristine model. All of these structures yielded ten IMI-AC adducts by using the PBE0-D3/6-31G* method, which predicts stable adducts at 0 K, and six of our models give negative free energies changes at room temperature. Thus, we expect that our IMI-AC models can be present when IMI interacts with an AC model. For one of the IMI-AC adducts, we applied solid-state techniques to avoid border effects, and we found that the imidacloprid is deprotonated giving reactive species, suggesting a new path to degrade this insecticide. Additionally, from this analysis, we proposed an additional IMI-AC adduct, which involves high free energy at room temperature. With this study, we show that our AC models can trap imidacloprid, which is quite convenient to remove this insecticide from our environment. Although it is well recognized that functionalized graphene structures are designed to trap some chemical compounds, to the best of our knowledge, this is the first time where IMI-graphene pieces interactions are studied in detail, and hydrogen bonds are analyzed through some scalar fields defined in quantum chemistry like the electron density and the non-covalent interactions index.
