The performance of density functional theory (DFT) (VWN-LDA, PBE-GGA, and B3LYP hybrid functionals), density-functional-based tight binding (DFTB), and ab initio methods [HF, MP2, CCSD, and CCSD(T)] for the treatment of London dispersion is investigated. Although highly correlated ab initio methods are capable of describing this phenomenon, if they are used with rather large basis sets, DFT methods are found to be inadequate for the description of H2/PAH (polycyclic aromatic hydrocarbon) interactions. As an alternative approach, an a posteriori addition of a van der Waals term to DFTB is proposed. This method provides results for H2/PAH interactions in close agreement with MP2 and higher-level ab initio methods. Bulk properties of graphite also compare well with the experimental data.

The aluminosilicate mineral imogolite is composed of single-walled nanotubes with stoichiometry of (HO)(3)Al(2)O(3)SiOH and occurs naturally in soils of volcanic origin. In the present work we study the stability and the electronic and mechanical properties of zigzag and armchair imogolite nanotubes using the density-functional tight-binding method. The (12,0) imogolite tube has the highest stability of all tubes studied here. Uniquely for nanotubes, imogolite has a minimum in the strain energy for the optimum structure. This is in agreement with experimental data, as shown by comparison with the simulated X-ray diffraction spectrum. An analysis of the electronic densities of states shows that all imogolite tubes, independent on their chirality and size, are insulators.

Halloysite is a clay mineral with stoichiometry Al 2 Si 2 O 5 (OH) 4 • nH 2 O that can grow into long tubules and is chemically similar to kaolinite. In this work we present a systematic study on the stability, electronic, and mechanical properties of zigzag and armchair single-walled halloysite nanotubes by means of the self-consistent charge density-functional tight-binding method (SCC-DFTB). The detailed analysis is focused on structural properties, strain energy, and band gap as a function of tube radii and Mulliken charge distribution. The strain energy of halloysite nanotubes does not have a monotonic character and the most stable structures should be observed in the region of radii above 24 Å, in agreement with experimental data. Analysis of the electronic density of states shows that all tubes are insulators. Our calculations predict that single-walled halloysite nanotubes have Young modulus in the same order of imogolite and inorganic nanotubes, but smaller than that of carbon nanotubes. Even though most of the properties are adequately described by simpler halloysite models, further studies on multiwalled and larger diameter tubes are in progress.

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Covellite (CuS) is an important mineral sulfide that can be used in many technological applications. It has a simple formula but a complex structure consisting of alternating layers of planar CuS3 triangles and CuS4 tetrahedrons with S-S bonds. Accurate first-principles calculations are performed for covellite structure (CuS), aiming to provide insights about its structural, mechanical and electronic properties and to unveil the nature of its chemical bonding. DFT and DFT+U methods have been used and showed to be sensitive to the correlation treatment (U value). Although it is not possible to extract a universal value of the U, this study indicates that U = 5 eV is an adequate value. The electronic structure analysis shows a significant metallic character due to p(S)-d(Cu) orbital interactions up to Fermi level. The projected density of states indicates that most of the contribution comes from the atomic orbitals in the [001] plane of the covellite, explaining the conductivity anisotropy observed experimentally. Topological analysis of the electron density was performed by means of quantum theory of atoms in molecules (QTAIM). Two different topological charges in Cu and S were calculated, confirming an ionic model with mix-charges. This mineral presents ionic degree of ∼ 32%. On the basis of the QTAIM analysis, the covalent character of S-S bond is confirmed, and the favored cleavage of CuS at the [001] surface might be at the Cu-S bond. The S atoms occupy most of the cell volume, and their contributions dominate the crystal compressibility: κ(S) ≈ κ(CuS).

Oxide-derived copper (OD-Cu) catalysts are promising candidates for the electrochemical CO 2 reduction reaction (CO 2 RR) due to the enhanced selectivity toward ethylene over methane evolution, which has been linked to the presence of subsurface oxygen (O sb ). In this work, O sb is investigated with theoretical methods. Although O sb is unstable in slab models, it becomes stabilized within a "manually" reduced OD-Cu nanocube model which was calculated by self-consistent charge density functional tight binding (SCC-DFTB). The results obtained with SCC-DFTB for the full nanocube were confirmed with subcluster models extracted from the nanocube, calculated with both density functional theory (DFT) and SCC-DFTB. The higher stability of O sb in the nanocube is attributed to the disordered structure and greater flexibility. The adsorption strength of CO on Cu(100) is enhanced by O sb withdrawing electron density from the Cu atom, resulting in reduction of the σ-repulsion. Hence, the coverage of CO may be increased, facilitating its dimerization.

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